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 rewrites the style to achieve compatibility with
inline_tab_audio_icons.css. compatibility only requires few small
non-affecting extra rules in inline_tab_audio_icons. This patch also
makes combined_favicon_and_tab_close_button work with themes where tab
background is not full opaque which is a nice bonus :)
