Freeze break request: End Session Dialog improvements

["Jasper St. Pierre" <jstpierre mecheye net>]

After the new system menus landed, we wanted to make improvements to the End Session Dialog to have more options, but never got around to finalizing the design and code until today.

This affects a minor feature of the system menu (hold alt to suspend has been reintroduced), and generally touches up on the end session dialog to fix bugs and refine the design.

https://bugzilla.gnome.org/show_bug.cgi?id=706612

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  Jasper