tweener: make timeline loop indefinitely

Tweener uses a clutter timeline to manage all active animations
running at a given moment.  The timeline is mopped up when no
animations are going any more.

Clutter requires timelines to have a finite duration, but since
animations can happen at any moment, no fixed duration can
accomodate the shell's needs.

To combat this problem, the tweener code picks a relatively
long duration: 1000 seconds. No string of animations should take
that long, so, in theory, that should be good enough.

Unfortunately, this tactic fails, in practice, when the user
suspends their machine, or VT switches.  An animation can take
much longer than 1000 seconds (~16 minutes) to complete in those
cases.  When the user resumes, or VT switches back the timeline
completes immediately (since it's already late) and tweener
never notices that the timeline stops ticking.

This commit changes the tweener timeline to automatically loop
back to 0 after completing, so that despite its fixed duration
property, it effectively never stops. Since the timeline loops,
its concept of elapsed time no longer increases monotonically,
so we now ignore it and track time ourselves with
GLib.get_monotonic_time().

This partially reverts commit
35764fa0.

https://bugzilla.gnome.org/show_bug.cgi?id=653833