gtkplacesidebar: use GtkListBox

gtkplacesidebar: use GtkListBox

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.