Follow-up I19da270a: Make overlay appear above non-VE skin items in Monobook

In I19da270a we bumped the z-index for the overlay from 4 down to 2, to avoid
clashing with the toolbar. However, the site logo, search box and personal
tools all have a z-index of 3, so 4 is required. Instead, bump the toolbar's
z-index up by 1 to 5.

Change-Id: I7e1edcf05cde054c7bcb8c13b5633930fb5ed3b5
Merge "Follow-up I19da270a: Make overlay appear above non-VE skin items in Monobook"