If you launched nmtui directly into the editor for a specific
connection, it would hang with a blank screen when you quit.
Fix this by changing the way startup works a bit, and have the created
toplevel NmtNewtForm get returned all the way to nmtui.c, which can
then connect to the "quit" signal on it and quit (rather than having
the different subprograms trying to guess whether they're supposed to
quit-on-exit or not).
