Add the "inner_item_margin" Theme constant to the Tree control

This PR adds the "inner_item_margin" Theme constant to the Tree Control.
It behaves like a horizontal padding (in CSS), but only in the active
writing direction (So on LTR it'll apply a left padding and on RTL right
padding).

The Editor Theme has been updated to make use of this and a result items
in Trees and ItemLists no longer "hugs" their border, expressing a proper
spacing instead.

doc/classes/Tree.xml

@@ -542,6 +542,18 @@
 		<theme_item name="icon_max_width" data_type="constant" type="int" default="0">
 			The maximum allowed width of the icon in item's cells. This limit is applied on top of the default size of the icon, but before the value set with [method TreeItem.set_icon_max_width]. The height is adjusted according to the icon's ratio.
 		</theme_item>
+		<theme_item name="inner_item_margin_bottom" data_type="constant" type="int" default="0">
+			The inner bottom margin of an item.
+		</theme_item>
+		<theme_item name="inner_item_margin_left" data_type="constant" type="int" default="0">
+			The inner left margin of an item.
+		</theme_item>
+		<theme_item name="inner_item_margin_right" data_type="constant" type="int" default="0">
+			The inner right margin of an item.
+		</theme_item>
+		<theme_item name="inner_item_margin_top" data_type="constant" type="int" default="0">
+			The inner top margin of an item.
+		</theme_item>
 		<theme_item name="item_margin" data_type="constant" type="int" default="16">
 			The horizontal margin at the start of an item. This is used when folding is enabled for the item.
 		</theme_item>

editor/editor_themes.cpp

@@ -1240,6 +1240,10 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 	theme->set_constant("h_separation", "Tree", 6 * EDSCALE);
 	theme->set_constant("guide_width", "Tree", border_width);
 	theme->set_constant("item_margin", "Tree", 3 * default_margin_size * EDSCALE);
+	theme->set_constant("inner_item_margin_bottom", "Tree", (default_margin_size + extra_spacing) * EDSCALE);
+	theme->set_constant("inner_item_margin_left", "Tree", (default_margin_size + extra_spacing) * EDSCALE);
+	theme->set_constant("inner_item_margin_right", "Tree", (default_margin_size + extra_spacing) * EDSCALE);
+	theme->set_constant("inner_item_margin_top", "Tree", (default_margin_size + extra_spacing) * EDSCALE);
 	theme->set_constant("button_margin", "Tree", default_margin_size * EDSCALE);
 	theme->set_constant("scroll_border", "Tree", 40 * EDSCALE);
 	theme->set_constant("scroll_speed", "Tree", 12);

scene/gui/tree.cpp

@@ -1652,6 +1652,10 @@ void Tree::_update_theme_item_cache() {
 	theme_cache.drop_position_color = get_theme_color(SNAME("drop_position_color"));
 	theme_cache.h_separation = get_theme_constant(SNAME("h_separation"));
 	theme_cache.v_separation = get_theme_constant(SNAME("v_separation"));
+	theme_cache.inner_item_margin_bottom = get_theme_constant(SNAME("inner_item_margin_bottom"));
+	theme_cache.inner_item_margin_left = get_theme_constant(SNAME("inner_item_margin_left"));
+	theme_cache.inner_item_margin_right = get_theme_constant(SNAME("inner_item_margin_right"));
+	theme_cache.inner_item_margin_top = get_theme_constant(SNAME("inner_item_margin_top"));
 	theme_cache.item_margin = get_theme_constant(SNAME("item_margin"));
 	theme_cache.button_margin = get_theme_constant(SNAME("button_margin"));
 	theme_cache.icon_max_width = get_theme_constant(SNAME("icon_max_width"));
@@ -1789,13 +1793,14 @@ int Tree::get_item_height(TreeItem *p_item) const {
 void Tree::draw_item_rect(TreeItem::Cell &p_cell, const Rect2i &p_rect, const Color &p_color, const Color &p_icon_color, int p_ol_size, const Color &p_ol_color) {
 	ERR_FAIL_COND(theme_cache.font.is_null());
 
-	Rect2i rect = p_rect;
+	Rect2i rect = p_rect.grow_individual(-theme_cache.inner_item_margin_left, -theme_cache.inner_item_margin_top, -theme_cache.inner_item_margin_right, -theme_cache.inner_item_margin_bottom);
 	Size2 ts = p_cell.text_buf->get_size();
 	bool rtl = is_layout_rtl();
 
 	int w = 0;
+	Size2i bmsize;
 	if (!p_cell.icon.is_null()) {
-		Size2i bmsize = _get_cell_icon_size(p_cell);
+		bmsize = _get_cell_icon_size(p_cell);
 		w += bmsize.width + theme_cache.h_separation;
 		if (rect.size.width > 0 && (w + ts.width) > rect.size.width) {
 			ts.width = rect.size.width - w;
@@ -1834,8 +1839,6 @@ void Tree::draw_item_rect(TreeItem::Cell &p_cell, const Rect2i &p_rect, const Co
 	}
 
 	if (!p_cell.icon.is_null()) {
-		Size2i bmsize = _get_cell_icon_size(p_cell);
-
 		p_cell.draw_icon(ci, rect.position + Size2i(0, Math::floor((real_t)(rect.size.y - bmsize.y) / 2)), bmsize, p_icon_color);
 		rect.position.x += bmsize.x + theme_cache.h_separation;
 		rect.size.x -= bmsize.x + theme_cache.h_separation;

scene/gui/tree.h

@@ -547,6 +547,10 @@ class Tree : public Control {
 
 		int h_separation = 0;
 		int v_separation = 0;
+		int inner_item_margin_bottom = 0;
+		int inner_item_margin_left = 0;
+		int inner_item_margin_right = 0;
+		int inner_item_margin_top = 0;
 		int item_margin = 0;
 		int button_margin = 0;
 		int icon_max_width = 0;

scene/resources/default_theme/default_theme.cpp

@@ -774,6 +774,10 @@ void fill_default_theme(Ref<Theme> &theme, const Ref<Font> &default_font, const
 	theme->set_constant("h_separation", "Tree", 4 * scale);
 	theme->set_constant("v_separation", "Tree", 4 * scale);
 	theme->set_constant("item_margin", "Tree", 16 * scale);
+	theme->set_constant("inner_item_margin_bottom", "Tree", 0);
+	theme->set_constant("inner_item_margin_left", "Tree", 0);
+	theme->set_constant("inner_item_margin_right", "Tree", 0);
+	theme->set_constant("inner_item_margin_top", "Tree", 0);
 	theme->set_constant("button_margin", "Tree", 4 * scale);
 	theme->set_constant("draw_relationship_lines", "Tree", 0);
 	theme->set_constant("relationship_line_width", "Tree", 1);