ve.init.mw.ViewPageTarget: Actually animate the toolbar during setup

There was a slideDown() call, but this didn't do anything since
toolbars are visible and in the DOM by default.

As a temporary hack, hide it synchronously after creation and
then do the slideDown still.

This could ever so briefly cause a flash, though that didn't
happen in my testing.

This makes the experience smoother when we initialise the surface.
In particular the moment where we swap #bodyContent for our Surface
(which should look visually almost identical), before this change
it was still a bit of a flash since the Surface version has a
toolbar on top, and thus instead of swapping smoothly, we hide
content and show a similar piece of content that has an incompatible
offset from the top.

Bug: 64751
Change-Id: Id94974ba71fd887ce494d7b2b16ec62d43b18575