AuthPrompt: properly remove user widgets

When replacing a user widget, we need to destroy the child, not just
unparent it, or it will leak.

https://bugzilla.gnome.org/show_bug.cgi?id=738256
gdm: disconnect signals

Many signal connections on global objects and on non-widgets were not
disconnected when the unlock screen was destroyed, causing leaks.

https://bugzilla.gnome.org/show_bug.cgi?id=738256
gdm: fix missing braces

Incorrect braces meant that if the ShellUserVerifier was destroyed before
the call to fprintManager.GetDefaultDeviceRemote(), the reply would result in
an error.

https://bugzilla.gnome.org/show_bug.cgi?id=738256
messageTray: add some missing signal disconnections

Disconnect some signal connections that weren't disconnected on actor destroy
(not the result of comprehensive review.)

https://bugzilla.gnome.org/show_bug.cgi?id=738256