Add priority to gizmos and fix small issues

User defined gizmos will haave higher preference than editor gizmos by
default. Also fixed some inconsistencies in the gizmos menu when using
custom gizmos.

editor/plugins/path_editor_plugin.h

@@ -62,6 +62,7 @@ protected:
 
 public:
 	String get_name() const;
+	int get_priority() const;
 	PathSpatialGizmoPlugin();
 };