NmtNewtForm is an NmtNewtWidget, but previously it was only realizing
its child, not itself, which is technically wrong (though it had no
noticeable effect until get_realized() was added).
Use newtPopWindowNoRefresh() rather than newtPopWindow() when
destroying a form, since often we have to destroy and then almost
immediately after re-create the same form, and we don't want that to
be visible.
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.)
The keyboard focus was getting reset to the first widget any time a
widget changed validity, because the form wasn't updating priv->focus
when it should have.
