Put category popups in the inner overlay for now

We really shouldn't need the inner overlay for this,
we should be able to deal with popups being in
oo-ui-window-overlay. But for now, we're not, and
this fixes the current problems.

Depends on If16d16d2b in oojs-ui.

Bug: 72052
Change-Id: Ie06056b96db19ac4caf1f9c0e3a1c49cfddc6682
Merge "Put category popups in the inner overlay for now"