popupMenu: Hide separators when they aren't separating

popupMenu: Hide separators when they aren't separating

A separator only makes sense if there are items on both
sides of it. There is quite a lot of code written
throughout the shell that manages the process of showing
and hiding separators as the items around those separators
change.

This commit drops all that code in favor of changes to the menu
implementation to dynamically hide or show separators as
appropriate, so the callers don't have to deal with it.

https://bugzilla.gnome.org/show_bug.cgi?id=657082