port many Qt shared pointers to standard smart pointers

There are a few benefits to using smart pointers from the standard library:
- std::unique_ptr has move semantics. With move semantics, transfer of ownership
can be properly expressed
- std::shared_ptr is more efficient than QSharedPointer
- more developers are used to them, making contributions for newcomers easier

We're also already using a mix of both; because Qt shared pointers provide
no benefits, porting to standard smart pointers improves consistency in
the code base. Because of that, this commit ports most of the uses of QSharedPointer
to std::shared_ptr, and some uses of QScopedPointer to std::unique_ptr