DesktopContext: Ensure that inspectors/menus are fully visible when created

* Make the PopupWidget that hosts the inspectors/menus non-clippable.
  It shouldn't be big enough to need it, and that makes it impossible
  to scroll it into view.

* Scroll the PopupWidget into view after updating its dimensions.

Depends on Ib07a8362 in OOjs UI. Without that patch the popup widget's
$element is a 0x0 box, so scrolling it into view doesn't make the
entire contents visible.

Bug: 52526
Change-Id: I4e49a466d5becdfd3f84aab11de915cfa12468fa