QApplication: refactor delivery and propagation of wheel events

Handle wheel grabbing via wheel_widget in a single place, and
propagate events in the same way for all (spontaneous) events.

Handle ScrollMomentum the same way as ScrollUpdate to allow
partial sequences.

Fix the incorrect ignoring of wheel events by default; like all
other input events, they are now again accepted by default and
ignored in the default event handler implementation of QWidget.
This way, implementing the handle suffices to accept the event.
Note that QWidget::wheelEvent doesn't need to be changed, as the
event is ignored there today (an oversight of the change made in
f253f4c3, perhaps).

This also fixes changing of direction of a wheel event while
the event sequence is grabbed by a widget.

Change-Id: Ia0f03c14dede80322d690ca50d085898a0497dbe
Fixes: QTBUG-67032
Task-number: QTBUG-79102
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

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