Improve EditorInspectorCategory

- Simplified the implementation and removed the hacks.
- Adapted the control to locale & theme changes.
- Made the context menu lazy-created.

editor/editor_node.cpp

@@ -938,6 +938,7 @@ void EditorNode::_notification(int p_what) {
 			use_system_accent_color = EDITOR_GET("interface/theme/use_system_accent_color");
 
 			if (EditorThemeManager::is_generated_theme_outdated()) {
+				class_icon_cache.clear();
 				_update_theme();
 				_build_icon_type_cache();
 				recent_scenes->reset_size();