Padding got lost when porting to GtkBox center widget because GtkBox does not support css padding.

As discussed on IRC, adding that support now is too risky, so wrap it in a GtkFrame that instead support padding etc. The padding defined in Adwaita now works as expected.


Unrelated to the patch: I do not see any code in gtkactionbar.c that deals with these widgets as "internal children"... I fear that gtk_container_get_children will return the revealer instead of the user widgets.