This also fixes two issues I found whilst writing the tests:
1. Exclude hidden items from the set of navigable buttons and
2. Exclude disabled items from the set of navigable buttons whilst navigating,
because they may get disabled in the meantime (like with the edit controls).
MozReview-Commit-ID: 5WThVoTZjbV
The height of the "panelmultiview" binding is now determined by the stack layout code, and doesn't have to be calculated manually via JavaScript anymore. This allows the removal of mutation and overflow observers, and reduces the number of synchronous layouts being made.
There is still a workaround included for wrapping blocks not being taken into account in height calculations.
MozReview-Commit-ID: 9rrPU5O5hUx
It's ok to cache the padding size of the main view and re-use it in this case,
because the padding is maintained for each view consistently.
The performance characteristics are therefore maintained.
MozReview-Commit-ID: GYT59NIiBET
This also improves the styling of checkbox buttons inside the menu and improves
support for adopting panelviews into the app menu from another area properly.
MozReview-Commit-ID: 1I9CeBx3zrz
