filechooserbutton: Don't show the (None) item when the combo box is popped up

filechooserbutton: Don't show the (None) item when the combo box is popped up

This is surprisingly tricky, since the (None) item *has* to be a visible item while
the combo box is *not* popped up, so that it can show its contents.  But the item
has to be *not* visible when the combo box is popped up.

Also, update the whole button's selection, not just the underlying dialog's, when
the combo box changes its selection - based on a patch by Paul Davis in
https://bugzilla.gnome.org/show_bug.cgi?id=691040#c20

Signed-off-by: Federico Mena Quintero <federico@gnome.org>