The places sidebar already does all the work to determine the
right label to use for the current location of the filechooser.
We want to use the same label in the headerbar subtitle, so
add some private API to get it.
We were using GTkTreeView in a simple list. Also, as we know,
GtkCellRenderers are not the best way to theme and manipulate
widgets.
So instead use a GtkListBox to modernize the GtkPlacesSidebar,
and in the way clean up some parts of the code (like headings)
which were not used anymore.
Also we don't use a model anymore, since the data is simple
enough to manage it in a subclass of the row itself.