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
Sometimes path nodes can survive longer than the style context that
created them. Don't crash in those cases.
Fixes startup of mutter.
Testcase included.
https://bugzilla.gnome.org/show_bug.cgi?id=746407
That test is not working anymore by design since commit 57c4f01e.
It was introduced pre-3.0.0 in commit 12d6b588 and the feature was never
utilized. So it seems safe to just remove the test.
