toolbar: don't call gtk_style_context_set_background()

toolbar: don't call gtk_style_context_set_background()

GtkToolbar doesn't have its own GdkWindow to draw on (it calls
gtk_widget_set_has_window(FALSE) in _init), but only an event window
(input-only).
Since gtk_widget_get_window() in that case will return the GdkWindow of
the parent container, by calling gtk_style_context_set_background() here
we're overriding the base background of the container instead of our.
While in most cases this doesn't have any noticeable effect, since
the toplevel GtkWindow will paint its background on top of it at the
beginning of the draw cycle, when the classic window hierarchy is
broken, such as when widgets are rendered through a clutter-gtk
offscreen embedding, the background will become visible, which is
undesirable.
Fix this by having GtkToolbar not call gtk_style_context_set_background
in its style_updated handler.