QmlJS: Speed up ValueOwner construction.

* Don't build all default values (including the global object)
  separately for each ValueOwner instance.
* Instead, keep all global, immutable values in a single, shared
  instance.

While refactoring, some cases where we *modified* the global object had
to be removed:

* C++ context properties no longer get injected into the global object,
  instead they now have their own scope just above the global one.
* The Qt object's prototype no longer gets modified in Link. Instead,
  it's now a reference to the "Qt" object provided in a qmltypes file.
* The whole concept of a function 'Activation' that could potentially
  affect the global object was removed.

Change-Id: Id382faf965efa747fcc7a9b0bc2c90429d84d61b
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>