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 "unhides" tabs scrollbox overflow indicators as well as adds
background for scrollbuttons when re-oredering tabs. Both of these are
pretty complicated things to achieve, so this patch also adds a small
compatibility layer for non_floating_sharp_tabs.css
