tabs: separators for reorderable tabs

https://bugzilla.gnome.org/show_bug.cgi?id=722366
Make box and grid transparent when insensitive

New Gtk started drawing the background for this containers and we have
*:insensitive wildcard rule which matches and causes problems.
Add them to the special cases along with image and label.
Maybe at some point we should remove the wildcard rule and do proper styling
for each widget

https://bugzilla.gnome.org/show_bug.cgi?id=721623