QWaylandAdwaitaDecoration: handle "fill:currentColor" CSS declaration

Some symbolic icon themes use a CSS declaration in the following
format: (style="fill:currentColor). Not handling this format we end up
with an invisible icon when dark style is used.

Backported on behalf of Joao Driessen <joaodriessen@gmail.com> from
QAdwaitaDecorations.

Pick-to: 6.12 6.11
Change-Id: I66e002cb9fd6f5c525d7805cd1929538668aab5c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>