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)

Change-Id: I8c9683dfe06ebb77c5342a26f08174b67e7cbd90
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>