Mark some QtMetaTypePrivate classes movable

These types are held in QVariant.

This change should be BC, because it's private API, even though
it is located in a public header file. The classes are used as
private data members in the respective public API classes, but
we don't change the size or composition, and holding these
types in a QList would be outside the use of non-private API
for which we promise BC.

Change-Id: I7ef1c1ca57e9d87c6474c97bb2fa8afef170c88f
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>