mw.Target: Fix document focus scroll bug

Follows-up I55ef2622c9eacc which activated code introduced in
mw.Target in commits before that one that caused a change in the
execution order.

Hiding of page content (regular wiki page content provided by
original view request) must happen before the surface document is
focussed.

We used to hide the content from mw.ViewPageTarget#setUpSurface,
which is called from #onReady, which focusses the document after
setUpSurface is done.

Most of this code was moved to mw.ViewPageTarget#onSurfaceReady
which is the listener for the surfaceReady event emitted from

If our surface document gets focus while the original wikipage
content container is still there, the view port is forced to
scroll down because our surface is the next element sibling after
the wikipage container in the DOM.

And browsers (apparently Chrome is not affected) naturally retain
scroll position even if the elements above the one you "scrolled to"
disappear.

We can't (and shouldn't) move the hidePageContent call because
that's the responsibility of the Target subclass, so instead
moved the document focus to below the hidePageContent which is
now also part of the responsibility of the Target subclass.

Also:
* Removed target.surfaceOptions reference because that property
  does not exist. We never passed a second argument here, and
  whatever this was intended for, doesn't exist.

Bug: 58089
Change-Id: I230fbd5401cbd6e3b9450c7f156650409be8ef16
Merge "mw.Target: Fix document focus scroll bug"