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 :)
decreasing the padding allows icon to be bigger. Modifying inline-margin
moves it closer to favicon and also pushes label towards it. This allows
the style to work better with default tab min width (76px) which is not
wide enough to fit favicon, audio icon and close button.
