Blender Git Loki

Previews: do not keep preview task and timer running continuously in 'Preview' mode.

The task was not so annoying (workers are just sleeping anyway), but having the timer
always checking for updates was not so nice. This remains quite minor optimization probably...

So now, we do not start anymore preview task when enabling previews. This is deferred
to the moment we really need it (i.e. when we cache block of entries currently visible).
Preview timer is started at the same time.

Further more, when preview task has been inactive for about one second (i.e. all
queued previews have been handled, and no new one have been queued), we free it
(switching to some kind of 'standby' mode).

This also allows to avoid stoping/restarting preview tasks on each redraw when user is scrolling.