QMimeData: Extract Method QMimeDataPrivate::find()

Centralize the search for the QMimeDataStruct with a given format in
find() (overloaded on const and non-const, but implemented only once),
and use it in the three other functions that performed lookup before.

In setData(), optimize the case where data is overwritten.

Use a std::vector to not have to think about hidden detaches when
implementing the const find() in terms of the non-const one.

Change-Id: I874e5c6ef9c97d98b42f29faccbc3043e8c6a855
Reviewed-by: David Faure <david.faure@kdab.com>