MultiPointTouchArea: handles mouse too, unless mouseEnabled is false

MultiPointTouchArea: handles mouse too, unless mouseEnabled is false

A new boolean property mouseEnabled is introduced (true by default).
If set to true, then it will handle any non-synthetic mouse event
as if it were a touch point.
If set to false, the area becomes transparent for real mouse events
so that a MultiPointTouchArea can be stacked on top of a MouseArea
in order to separate handling of touch and mouse.
In either case it continues to absorb and ignore synthesized mouse
events (including touch-to-mouse synthesis in QQuickWindow).

[ChangeLog][QtQuick][MultiPointTouchArea]handles mouse as a touchpoint;
added mouseEnabled property to permit transparent pass-through to
mouse-sensitive items

Change-Id: I4af94d838f0060154494589c0f15c6858ee89ddb
Task-number: QTBUG-31047
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>