Bug 1870803 introduced a transition to window activeness visuals. But
the lowered opacity breaks the z-index hackery we use to let
window-controls show through nav-bar.
This patch basically undoes the changes from bug 1870803
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 way we can handle maximized windows not having the other drag space
like Firefox does normally. This patch additionally adds window control
width values for Win7 Win8 and linux