src/shutdown-dialog.vala: implement timer function triggering focused button.

Once the shutdown dialog is created/realized/shown, a new message will
show up at the bottom of the dialog signifying that the default action
will be triggered automatically in a configurable amount of seconds.

This message will be updated once per second, counting down.

Once the countdown reaches zero, the currently focused button (if any)
is automatically clicked.

The message will keep showing for another 10 seconds, just in case the
user removed the focus and no button is actually focused.

Multiple actions will cancel the timer:
  - Pressing escape.
  - Selecting a different button via (Shift +) Tab or the left or right
    arrow keys.
  - Clicking within the dialog (but not on any button).
  - Closing the dialog.

Fixes: https://github.com/ArcticaProject/arctica-greeter/issues/39

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