Make editor use translation domains

How editor plugins use this feature:
1. Pick a unique translation domain name.
2. `_enter_tree()`: load translations into that translation domain.
3. Call `set_translation_domain()` for its root UI node.
4. `_exit_tree()`: remove that translation domain.

Plugins can also set the translation domain to `godot.editor` for
nested nodes that should use editor translations. `EditorFileDialog`
automatically does this.

editor/editor_node.cpp

@@ -6557,6 +6557,8 @@ EditorNode::EditorNode() {
 	DEV_ASSERT(!singleton);
 	singleton = this;
 
+	set_translation_domain("godot.editor");
+
 	Resource::_get_local_scene_func = _resource_get_edited_scene;
 
 	{