window-slot: split a function

We're going to use this code later.
window-slot: reset extra location widgets on view switching

Since extra location widgets get references to a view - in particular
NautilusTrashBar - we should make sure to destroy and recreate them
together with the view.
view: always send selection change signals

For some reason, we're complicating the view selection code here; this
causes bugs e.g. when switching views, since actions that depend on the
selection being available will not be correctly updated.