Implement a good part of QJSValue using V4.
Keep the QV8Engine for now (as it is used everywhere),
but add an ExecutionEngine pointer to it so other parts
can more easily be ported to V4.
Remove a lot of other stuff that's not required anymore.

Change-Id: Ibe2c9ab10f0ee977e0e3d05a42f526206d7b22b5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>