QMouseEvent: add constructor which takes the source as a parameter

Simplify the code by passing the source of a mouse event
directly to the constructor instead of setting it by
QGuiApplicationPrivate::setMouseEventSource().

Change-Id: I1774cf39a211d36d3adf0ff30f3bd2fb7c5fb429
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>