Fix GC issue with incubators

Never use multiple inheritance with Managed subclasses,
as this can easily mess up garbage collection. In this
case the vtable from the QQmlIncubator would be added
before the start of the Managed pointer. That would
not work correctly for the memory manager that casts
void pointers to Managed pointers.

Change-Id: I1c1ebc6c44bd9cb77eea49738e86ce3374c7ef80
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix some more issues with exact GC

Get the formal and local names of function object from the
compilation unit to avoid creating another set of strings.
Use a ScopedFunctionObject in eval()

Change-Id: I6693aec2e88818df9c160b3780db12d8df79c2fe
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>