Don't rely on QQuickWindow delivering QEvent::Ungrab as mouseUngrabEvent (as...

Don't rely on QQuickWindow delivering QEvent::Ungrab as mouseUngrabEvent (as it no longer does in Qt 5.8+)

Summary:
QQuickWindow::sendEvent in Qt 5.7 and older had the following code
to deliver QEvent::UngrabMouse by calling QQuickItem::mouseUngrabEvent:

case QEvent::UngrabMouse: {
        QSet<QQuickItem *> hasFiltered;
        if (!d->sendFilteredMouseEvent(item->parentItem(), item, e, &hasFiltered)) {
            e->accept();
            item->mouseUngrabEvent();
        }
    }

This is gone from Qt 5.8+. While QEvent::Ungrab is still delivered to
items, QQuickItem::mouseUngrabEvent is only called under constrained
circumstances elsewhere, e.g. when ending an actual mouse grab held by
an item and tracked by Qt.

MouseEventListener relied on mouseUngrabEvent being called to implement
something akin to MouseArea::canceled: Signaling a user it should clean
up state after a press event, instead of, say, assuming the button is
still held and waiting around for a release event. While QEvent::Ungrab
was already being intercepted as well, it was only done for event de-
duplication, not used for the above.

This changes the code so handleUngrab checks first whether we're
actually in press state (to make it safe to call repeatedly) and then
call it from both the generic event handler and mouseUngrabEvent. This
makes it work again with newer Qts.

We rely on this particularly in the desktop containment, where we use
EventGenerator from this same lib to deliver QEvent::Ungrab to applets
when the containment goes into applet move mode on press-and-hold.
Without MouseEventListener emiting canceled in response, e.g. moving
a Folder View applet will e.g. put it into rectangle selection mode
unwanted.
BUG:380354

Reviewers: #plasma

Subscribers: plasma-devel, #frameworks

Tags: #plasma, #frameworks

Differential Revision: https://phabricator.kde.org/D6246