Node3D gizmo improvements

* Clean-up of node_3d_editor_plugin.{h,cpp}: removed unused code, fixed some bugs.
* Moved node_3d_editor_gizmos.{h,cpp} to editor/plugins.
* Added support for multiple gizmos per node. This means custom gizmos will no longer override the built-in ones and that multiple gizmos can be used in more complex nodes.
* Added support for handle IDs. When adding handles to a gizmo, an ID can be specified for each one, making it easier to work with gizmos that have a variable number of handles.
* Added support for subgizmos, selectable elements that can be transformed without needing a node of their own. By overriding _subgizmo_intersect_frustum() and/or _subgizmo_intersect_ray() gizmos can define which subgizmos should be selected on a region or click selection. Subgizmo transformations are applied using get/set/commit virtual methods, similar to how handles work.

scene/3d/visible_on_screen_notifier_3d.cpp

@@ -63,7 +63,7 @@ void VisibleOnScreenNotifier3D::set_aabb(const AABB &p_aabb) {
 
 	RS::get_singleton()->visibility_notifier_set_aabb(get_base(), aabb);
 
-	update_gizmo();
+	update_gizmos();
 }
 
 AABB VisibleOnScreenNotifier3D::get_aabb() const {