This gives a GtkSettings object for resolving system-dependant things -
like the default font family and font size.
No code does this yet, but we have an API.
Only GtkSettings implements this.
Make _gtk_style_provider_private_get_color() return a GtkCssValue (a
GtkCssColorValue to be exact) instead of GtkSymbolicColor.
With this, the symbolic color usage inside GTK is minimized.
This way, we don't have to do magic inside GtkStyleContext, but have a
real API.
As a cute bonus, this object implements GtkStyleProvider itself. So we
can just pretend there's only one provider.