Track target widget when wheel events are received

This issue is reproducible on OS X when using a Magic Mouse
or a combination of Magic Trackpad and regular mouse. In these
cases it's possible to start a scrolling gesture on one widget
and move the mouse cursor over another widget.

Although we send the wheel event phase information, we never
made any use of it. This means that a widget would start
scrolling even though it never received a ScrollBegin event.

In this patch, we make sure the scrolling cycle is respected
and that once a widget starts scrolling, it'll be recieving
all the wheel events until a ScrollEnd event reaches the
application.

For those input devices not supporting a proper phase cycle,
we introduce a new (undocumented) phase value, NoScrollPhase.
If the wheel event phase is NoScrollPhase, then we ignore
the current scroll widget and proceed as usual. This value
is the default for wheel events. It's up to the platform
plugin to set the proper phase value according to the data
received from the OS.

Finally, we fix a few of QWheelEvent constructors to properly
initialize the phase and source properties.

Task-number: QTBUG-50199
Change-Id: I3773729a9c757e2d2fcc5100dcd79f0ed26cb808
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

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