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
Editor: Warn if title change is unsaved

closes #3643
- TitleScratch is compared with Title; if there’s a difference, we have
unsaved changes and open up the modal.
Do not loop keyboard nav on content list.
Activate specific tab that is tapped/clicked.
Merge pull request #3650 from felixrieseberg/iss3643-2

Editor: Warn if title change is unsaved
Merge pull request #3628 from novaugust/popover-btn-binding#3603

Add open class to popover buttons
Merge pull request #3655 from rwjblue/dont-loop-keyboard-nav

Do not loop keyboard nav on content list.
Merge pull request #3656 from rwjblue/activate-specific-tab-not-toggle

Activate specific tab that is tapped/clicked.