Fix TapHandler so that it actually registers a tap
This bug caused all quick examples that used the shared\LauncherList.qml to be broken.
In QtGui, QSinglePointEvent will construct itself with a point id of 0 if there is a valid point, and with a point id of -1 if the point is invalid (the default constructor does the latter). However, QQuickSinglePointHandler::wantsPointerEvent() did not agree with that, because it assumed that a point id of 0 meant uninitialized/invalid point. The fix is to change QQuickSinglePointHandler::wantsPointerEvent() and QQuickHandlerPoint so that it assumes that the id -1 is now an invalid point, (instead of 0)
I'd like to backport this because KWin internally can have touch points with id 0 which means things break if those are passed to Qt.