This change achieves multiple things:
* The sizing options for placeholder are in one place
* Improves compatibility with other styles
* Improves compatibility with other window control placements
* Makes fullscreen work
* Makes menubar not take any space when disabled
Previously, when Firefox did not have :has selector support, we needed
to create :hover triggered space to activate menubar, this took some
space from toolbar and required special styling to support translucent
toolbar background colors. But it was necessary, because otherwise there
would have been no way to access menubar items.
This still assumes that the user has menubar disabled, but that is fine,
because if menubar is enabled, then there is no reason to use this style
in the first place.
Flexbox model will replace old xul box as the default display model so
this patch adds support for that in a whole bunch of styles.
A lot of style rules are marked as "Fx < 112 compatibility" rules and
those can be removed when 112 hits release.
