Implement Particle Trails

-Enable the trails and set the length in seconds
-Provide a mesh with a skeleton and a skin
-Or, alternatively use one of the built-in TubeTrailMesh/RibbonTrailMesh
-Works deterministically
-Fixed particle collisions (were broken)
-Not working in 2D yet (that will happen next)

editor/scene_tree_editor.h

@@ -157,6 +157,8 @@ public:
 
 	Tree *get_scene_tree() { return tree; }
 
+	void update_warning();
+
 	SceneTreeEditor(bool p_label = true, bool p_can_rename = false, bool p_can_open_instance = false);
 	~SceneTreeEditor();
 };