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
This is done because we want the button background to fill toolbar
height, but we don't actually know how tall the toolbar is and thus
working with list-style-image + padding is going to produce stretced
icon on some systems.
This lets default light theme to use toolbar-bgcolor for the selected
theme as well as make the line atop of selected tab (with default light theme) show as blue as per
Photon styling.
that should have already been the case previously, but if one were to
use some other custom style that added .tab-background::before pseudo
element, then all tabs would have a playing animation which would have
trashed performance.
