Pin autopause when no longer visible

[rish987]

Following #126, when an infoview is closed, it should notify its info, which will keep track of how many visible infoviews it is attached to. If that number goes to zero, it should then notify all of its pins, which likewise keep track of how many visible infos they are attached to, and when that goes to zero for a pin, the pin should (configurably) automatically pause itself.