Use GtkCssValue in symbolic color resolving

Use GtkCssValue in symbolic color resolving

We now store the symbolic colors as a GtkCssValue which means that
we can reuse the color when resolving and storing the color in
the computed values in the style context.

Additionally we keep a last_resolved GtkCssValue cache in the
GtkSymbolicColor, and if resolving the color returns the same as
last time we reuse the old value. This further increases sharing
of Css Values.