QtProtobuf: update and fix type registration

Currently the type registration is manual, as in, you need to manually call a function to register types on startup of your program. The way it is currently done may also lead to some warnings in Qt as qtprotobuf attempts to register aliases to types about re-registering types (which was fine in qt5, but in qt6 it is resolved to the actual type). And one assert about a call to qRegisterNormalizedMetaType with a non-normalized type, which should all go away.

Preferably the types should just register themselves without adding (too much) overhead to startup times.

The code is still in git.qt.io