Fix using tablet on QML widgets

In previous versions of Qt (wintab impeplementation) the events were
marked by Qt::MouseEventSynthesizedBySystem flag only when they were
synthesized from touch, not from tablet events. This is what
QWindowsTabletSupport does and what QQuickWindow expects (it
filters out all synthesized events). This patch recovers the old behavior
for the new QWindowsPointerHandler tablet API implementation.

CCBUG:406668