I've created a draft implementation of a tree view[1], as I thought it's
easier to decide whether it's useful if there is a working
implementation to play with.
I've added auto-expand for dnd which works with the sidebar tree view,
too. This patch may solve some of the problems some users have with
spatial nautilus.
There are probably still some rough edges needing cleanup but it doesn't
work too bad so far. Besides that, expansion state saving and loading
have yet to be added for spatial windows.