Clean up Target properties

Move target.surface from mw.Target to Target
* All targets use this, let's standardise it.

Move target.$document from mw.ViewPageTarget to Target
* It was initialised with null in mw.ViewPageTarget, but the
  assignment happened in mw.Target. So it should be moved up
  at least to mw.Target.
* Since it is useful to have in sa.Target as well, moved it up
  to the abstract Target, and implemented in sa.Target and
  immediately used in the standalone demo where we were already
  duplicating the find( '.ve-ce-documentNode' ).

Add missing target.setupDone = false; in sa.Target

Add missing target.toolbar to Target
* Was used in all subclasses, but never initialised in any of
  the constructors. Let's standardise this property name as well
  (instead of initialising it in three places).

Move target#event-surfaceReady from mw.Target to Target
* sa.Target uses it as well, and considering Platform#initialize
  is already standardised in the abstract class, Target#setup
  being deferred is most likely to happen in each target as well
  so let's avoid different events being invented for the same
  thing and consistently use 'surfaceReady'.

Change-Id: Ia8bde188a4cde7e1615c2ae9c5b758eefc5d9cb7