style cascade: Allow cascades with more ancestors

style cascade: Allow cascades with more ancestors

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