Add tooltip popup as a child of p_target to avoid jitter

Previously the tooltip popup was added as a child of the `p_target`
viewport (usually the root window), which caused the root window to
recalculate the window size, which could cause jitter issues on i3wm
if the actual width of the root window was less than `1024`.

editor/editor_help.cpp

@@ -3868,7 +3868,7 @@ void EditorHelpBitTooltip::show_tooltip(EditorHelpBit *p_help_bit, Control *p_ta
 	EditorHelpBitTooltip *tooltip = memnew(EditorHelpBitTooltip(p_target));
 	p_help_bit->connect("request_hide", callable_mp(tooltip, &EditorHelpBitTooltip::_safe_queue_free));
 	tooltip->add_child(p_help_bit);
-	p_target->get_viewport()->add_child(tooltip);
+	p_target->add_child(tooltip);
 	p_help_bit->update_content_height();
 	tooltip->popup_under_cursor();
 }