Add QOwningBiPointer class

QBiPointer does not own the object it points to; however, in quite a few places it would make more sense for it to actually own it.

So introduce
template<
class T1,
class T2,
class Deleter1 = std::default_delete<T>,
class Deleter2 = std::default_delete<T>
> class QOwningBiPtr;

with the obvious semantics.

Use the chance to reconsider the API (as<T> instead of asT1 and asT2, non asserting as, more?)