endSessionDialog: List other users and sessions in with the inhibitors

Instead of in a separate dialog. This does not meet the designs right
now, but it's a good first start.

https://bugzilla.gnome.org/show_bug.cgi?id=706612
endSessionDialog: Convert to the standard _sync pattern

... for starting and stopping the timer. This helps clean up the
state transitions in the code when caring about multiple things.

https://bugzilla.gnome.org/show_bug.cgi?id=706612
endSessionDialog: Don't stop the timer when we have inhibitors

https://bugzilla.gnome.org/show_bug.cgi?id=706612
endSessionDialog: Split into two sections

https://bugzilla.gnome.org/show_bug.cgi?id=706612
system: Add a way to suspend from the system menu

When we implemented the new designs, we lost the ability to suspend
from the system menu. Re-enable this ability by re-adding the hidden
"Alt" shortcut item.

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