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.
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