Fix overlay for Monobook

Because of the z-indexes of major elements of the mono book
skin, and because the overlay containers are appended to the
body, the overlays can't be positioned between the surface and
the toolbar. Before this fix, the overlays are appearing beneath
the surface. This fix will retain proper positioning of the overlays
between the surface and the toolbar for Vector, and will overlay
everything in Monobook.

Later, we will have the overlay container more tightly integrated
with the surface to avoid this stacking problem.

Change-Id: Ibb1553099cc1e35e6a0928a99b584885508ca5b6