Add move constructor and move assignment operator.
To allow for the the change to be implemented QTemporaryDir, the
internal d_ptr was modified from a QScopedPointer, which is not movable,
to a raw pointer.
Add member + free swap implementations.
Add tests.
[ChangeLog][QtCore][QTemporaryDir] Enabled move semantics.
Change-Id: I9f196a77c70b4ca0b7f0c06505d00fdd87a9785c
Reviewed-by: Marc Mutz <marc.mutz@qt.io>