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/mesh_instance_3d.cpp

@@ -133,7 +133,7 @@ void MeshInstance3D::set_mesh(const Ref<Mesh> &p_mesh) {
 		set_base(RID());
 	}
 
-	update_gizmo();
+	update_gizmos();
 
 	notify_property_list_changed();
 }
@@ -356,7 +356,7 @@ Ref<Material> MeshInstance3D::get_active_material(int p_surface) const {
 void MeshInstance3D::_mesh_changed() {
 	ERR_FAIL_COND(mesh.is_null());
 	surface_override_materials.resize(mesh->get_surface_count());
-	update_gizmo();
+	update_gizmos();
 }
 
 void MeshInstance3D::create_debug_tangents() {