Gijs for front-end bits, layout for the new CSS properties and the
removal of nsDeckFrame / nsStackLayout, Jamie and Morgan for the a11y
changes.
As discussed in the bug, the main tricky part here is handling a11y
correctly. For <deck>, that's trivial (just use `visibility: hidden` to
hide the panels visually, while removing the unselected panels from the
a11y tree).
For <tabpanels> however we need to do something special. We do want to
hide stuff visually, but we want to preserve the contents in the a11y
tree.
For that, the easiest fix is introducing a new privileged CSS property
(-moz-subtree-hidden-only-visually), which takes care of not painting
the frame, but marks stuff offscreen in the accessibility tree. This is
not intended to be a property used widely.
Other than that, the changes are relatively straight-forward, though
some of the accessible/mac changes I could get a sanity-check on.
Differential Revision: https://phabricator.services.mozilla.com/D157875
