Symbolic Icons / SVG Icons that get recoloured

[David Edmundson]

Turning the list of differences into an actionable TODO list the
following changes would be needed by both parties to have exact
feature parity:

GTK changes:
 - Add 2 new CSS classes "bg", and "highlight"
 - apply SVG CSS injection to include non-symbolic icons. On icons
that don't end in "-symbolic" we should set the foreground colour on
the .fg class not to all SVG paths

KDE/Qt changes:
 - apply foreground colour to all SVG paths when icon name ends in "-symbolic"
 - rename our class names to use the same semantic names
 - support "!important" in QtSVG

>From a KDE side both myself and Marco are happy to make these changes,
I would like to hear back from the GTK side.