Remove all remaining GParamSpec name and blurb strings (and fix
indentation while we're there), and add G_PARAM_STATIC_STRINGS to all
paramspecs that were lacking it.
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).
The main "connect" and "edit" windows set the "escape-exits" flag, but
that just closed the form without exiting the app, leaving the user
trapped. Fix this by emitting a signal when the form quits, and
catching that. (And now we don't need to watch the "clicked" signal on
the quit buttons, since they have the "exit-on-activate" flag set.)
