QCollatorSortKey: add some std::move

All implementations of QCollator::sortKey() can benefit from
moving their CollatorKeyType into the QCollatorSortKeyPrivate
on construction.

So make the QCollatorSortKeyPrivate ctor a perfect forwarder
for its m_key member, and add std::move() calls where they
were missing (in all but one case, lvalues were passed).

Make the ctor explicit, as it should have been from the
beginning.

Change-Id: I2a1cdda5fd23990ace019b963df895c621a1fa85
Reviewed-by: Lars Knoll <lars.knoll@qt.io>