Rename View2D to View

As we approach 3.0 final, it is clear that we will not have a 3D view in the near future.  It is awkward to have `ol.View2D` without `ol.View3D`, so we have chosen to say instead that `ol.View` is 2D by definition.  In the future, we may add an `ol.Camera` object to provide a 3D perspective.

This is a major breaking change (and we shouldn't expect more of this magnitude before the final release).  It should be fairly straightforward for application developers to upgrade after this change.  Assuming applications are only using exportable symbols, here are the required changes:

 * rename `ol.View2D` to `ol.View`
 * remove any `getView2D` calls (use the view directly instead)
 * rename `frameState.view2DState` to `frameState.viewState`