Provide QEventStorage to allow storing QEvent subclasses without exposing the copy operations as public API

This is a follow-up on QTBUG-99563. The UB there is fixed, but we still use QMutable*Event to store event (copies) for later reuse, so the QMutable*Event classes can't, yet, be turned into a pure namespace class, like QTBUG-99563 requested.

Acceptance criteria:

• Add QEventStorage (or similar) as private API
• Port all remaining users of QEvent copying over to it
• Make QMutable*Event a stateless namespace for static gettters and setters.