Get rid of the first half of QQmlGuard usage

QQmlGuard was being used as a more performant replacement
for QPointer. QPointer got now fixed in Qt 5.0, making this
class obsolete.

Change-Id: I77eef066c4823af4b074ef32d4d78dbd67c36cd0
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>