Make icons of scripted and custom classes fit the editor UI

Also:
- Add an option to limit the icon size in PopupMenu.
This is similar to how this works in Tree and TreeItem.
- Add the same option to TabBar.
- Add a theme constant for Tree, PopupMenu, Button, and
TabBar to apply this limit on the control level.

Co-authored-by: Daylily-Zeleen <daylily-zeleen@foxmail.com>

editor/scene_tree_editor.cpp

@@ -878,6 +878,8 @@ void SceneTreeEditor::_notification(int p_what) {
 		} break;
 
 		case NOTIFICATION_THEME_CHANGED: {
+			tree->add_theme_constant_override("icon_max_width", get_theme_constant(SNAME("class_icon_size"), SNAME("Editor")));
+
 			_update_tree();
 		} break;
 	}