Add open class to popover buttons

Ref#3603
- Popover Buttons now have an `open` class applied to them when their popover is open
- The open class is removed from the popover button at the start of the popover's fadeout
- Consolidated common code into popover-mixin
Merge pull request #3628 from novaugust/popover-btn-binding#3603

Add open class to popover buttons