Split off setup from the rest of mw.ViewPageTarget

Initialisation initialisation? It's time to rename ve.init
to ve.platform and ve.init.Platform to ve.platform.Environment,
but that'll come later.

* Moved support detection and skin set up to separate class-less
  file.
* Swapped usage of ve.msg for mw.msg.
* Callback of edit tab now does an mw.loader call to fetch
  the actual VisualEditor libraries.
  Though mw.loader won't load the same thing twice, we would
  bind a callback each time. To avoid instantiating ViewPageTarget
  more than once we use a Deferred.

Bug: 50542
Bug: 50608
Bug: 50612
Change-Id: Ic8b0004ab5288fa91bb29d496485b93ffd8d977e
Merge "Split off setup from the rest of mw.ViewPageTarget"