Tweak pressed CheckBox appearance in the editor

This uses the accent color to match pressed CheckButtons after they
were updated. Checked checkboxes are now more prominent in the user's
peripheral vision, which can be useful at times. This also matches
how checkboxes look in most operating systems and web browsers.

(cherry picked from commit 0d27c4d)

editor/editor_themes.cpp

@@ -217,6 +217,7 @@ void editor_register_and_generate_icons(Ref<Theme> p_theme, bool p_dark_theme =
 		exceptions.insert("ProceduralSky");
 		exceptions.insert("EditorControlAnchor");
 		exceptions.insert("DefaultProjectIcon");
+		exceptions.insert("GuiChecked");
 		exceptions.insert("GuiCloseCustomizable");
 		exceptions.insert("GuiGraphNodePort");
 		exceptions.insert("GuiResizer");