Allow phantoms and focus areas to be overridden

And actually use this functionality on MWBlockImage to
highlight centred images correctly.

FocusableNode
* Actually use $focusable to render the highlight correctly

ProtectedNode
* Allow a $phantomable element to define which element to
  highlight on mouseenter
* As phatoms are built off shields, make sure a shield is
  given to $phantomable

RelocatableNode
* Allow a $relocatable element to define which element the
  relocatable marker is measured against

ui.Context
* Use $focusable (when available) for positioning the popup

Plus some documentation fixes

Change-Id: I370337239af4fc935cd86757b3ce03011bae5ba8
Merge "Allow phantoms and focus areas to be overridden"