Changed QML type locking

Changed QML type locking

Replace the use of QReadWriteLock with a plain QMutex. At this point
QReadWriteLock adds a lot of overhead due to its internal QHash and otherwise
offers little over a recursive mutex, especially given that there is most
of the time only one reading thread and one writing thread.

Change-Id: I0a91e4a64cff5d25e4a133ed46b08912e81bb055
Reviewed-by: Lars Knoll <lars.knoll@digia.com>