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.
There is still one(known?) case that happens if one uses a theme with
non-opaque toolbar background-color with optional extra-border rules.
But that option isn't too nice with non-opaque colors anyway so left as
todo.
In addition, this patch makes the optional rules use @supports
-moz-bool-pref rules instead of having to remove commented sections in
the css file.
This patch makes outward curves use a sliver of toolbox background-image
to mask some artifacts that are results of several semi-transparent
surfaces. This also changes the curve svg image a bit to makes
curve-border transitions more smooth.
With positioninnedtabs, the left-most tab might end up outside of the
window under some circumstances (for example when menubar is enabled).
This patch moves tabbrowser-tabs away from the window edge equal to the
curve-size and makes the pre-tabs spacer a bit narrower to make the
space not look too large when the spacer is shown.
This patch also adds some comments for svg.context-properties.content.enabled
which is a requirement as well as make default theme have separate
background-color for curves.
