Fixes#350 by applying workaround for titlebar will-change and opacity
layering issues
Makes the menubar menu coloring work on default theme
Make menubar bottom-margin computation use the same values as menubar
height - this should make other toolbars stay in place without moving
up/down on most setups, probably.
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.
