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.
This patch makes the style work together with non_floating sharp tabs
which is now a requirement for this style, because this style won't make
sense with proton-like tabs.
