Previously a style cascade's parent could not have a parent itself. That
represented the two levels at which you could add a style provider: at
the screen level, with gtk_style_context_add_provider_for_screen(), and
at the style context level, with gtk_style_context_add_provider().
This commit changes no functionality, but this change will be necessary
for adding style providers in the future that apply to a subtree of the
widget tree. It relaxes the requirement that a style cascade's parent
must not have a parent, since in the future a style context may be
affected by any number of parent widgets' style contexts.
https://bugzilla.gnome.org/show_bug.cgi?id=751409
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.
