Unused self parameters in Q*Application methods

In QCoreApplication, QGuiApplication, and QApplication, static methods are handled in three different ways, arbitrarily:

1. No self parameter
2. &self
3. Pin<&mut Self>

The first approach is the only correct one. The second approach has an unused parameter, but at least it doesn't do anything with it. The third approach adds pinning and mutability logic that, again, is completely unused. It's passing around and wrapping and unwrapping a mutable pointer to an object that nothing happens to.

Obviously, removing those parameters would cause breaking changes, but I think it should probably be done in time for 1.0.