QMenu: make less sensitive to mouse jitter

On systems where a right-press brings up the menu, the next mouse
move will select an action, even if it's just a move by a single pixel.
This makes it too easy to activate an action on e.g a context menu
accidentially when the button is released.

Ignore the first couple of mouse moves, using the same logic that
prevents accidental tearing off.

Change-Id: Ib4dd448ef2d6ae915b48da62666aa95b37145d63
Fixes: QTBUG-57849
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/widgets/widgets/qmenu.cpp

@@ -3473,7 +3473,7 @@ void QMenu::mouseMoveEvent(QMouseEvent *e)
         return;
 
     d->motions++;
-    if (d->motions == 0)
+    if (!d->hasMouseMoved(e->globalPosition().toPoint()))
         return;
 
     d->hasHadMouse = d->hasHadMouse || rect().contains(e->position().toPoint());