Moved plugins and refactor command system

- Moved plugins to apropriate sub folders
- Refactor command system with new add_command method and rename GetCommandSystemEvent to GetNewCommandSystemEvent
- Add RegisterCommandEvent for dynamic command registration
- Change footer container orientation to VERTICAL
- Add search-highlight tag to source buffer
- Add file change detection (deleted, externally modified) in source_file
- Add JSON prettify option to source view popup menu
- Enable hexpand on VTE widget
- Update plugins_controller_mixin to use widget_registry

src/plugins/plugins_controller_mixin.py

@@ -14,10 +14,7 @@ class InvalidPluginException(Exception):
 class PluginsControllerMixin:
 
     def requests_ui_element(self, target_id: str):
-        builder   = settings_manager.get_builder()
-        ui_target = builder.get_object(target_id)
+        if not target_id in widget_registery.objects:
+            raise InvalidPluginException('Unknown UI "target_id" given in requests.')
 
-        if not ui_target:
-            raise InvalidPluginException('Unknown "target_id" given in requests.')
-
-        return ui_target
+        return widget_registery.objects[target_id]