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.
browser-bottombox doesn't exist anymore so create space using
body::after instead. This patch additionally fixes various alignment
issues with elements in the statusbar.
Normally web-content hides the real statusbar but with RFP letterboxing
that might not happen. So lets inherit the background-color from
tabbrowser-tabpanels to make sure it's covered properly.
