gdm: honor timed login delay even if animations disabled

This is a downstream patch I apparently never upstreamed ? weird.

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gnome-shell currently initiates an automatic login attempt if timed login is enabled and the timed login animation completes.

Unfortunately, if animations are disabled (as is the case for virtual machines) then the timed login animation will complete instantly, and timed login will proceed immediately after gnome-shell has noticed the user is idle for 5 seconds.

This commit addresses that problem by initiating timed login and the animation from a main loop timeout, instead of using the tweener api.