Currently the node tools asset tree for extending 3D view headers is
rebuilt whenever it's cleared, which is done by clearing the tree's storage.
That means the data is rebuilt on every redraw if there are no node tools
and the tree storage is empty. With larger asset libraries that can be
quite expensive.
Now, instead of clearing the map, use a new dirty tag to store whether
the asset tree is out of date compared to the assets.
This should resolve#120494
52cff75ce0