Snackbar plus page Visbility API

[Garbee]

It would be nice to use the page visibility API to control whether to show a snackbar or not. We can at least check this before showing new snackbars, and continue the queue once visible again. Not sure if we can stall the timer based on page visibility on currently showing snackbars. Will research more as it is upgraded.

[addyosmani]

My gut feeling having looked at sites using the snackbar is that this might be best left to the developer to decide whether snackbar creation should factor in page visibility. That said, open to more research here <3