Fix Flickable mouse grab issue.

This issue cannot be reproduced on the desktop with a mouse (at least I
could not).  It can however reliably be reproduced on iMX6 embedded hardware
with a touch-screen.  It seems like the TS driver is reporting mouse events
with a much more coarse granularity than the USB mouse driver, which in turn
triggers this bug.
(Qt4's declarative, as well as Qt5's quick1 and 2 all have the same issue)

Change-Id: Id151e3847bc8d77c7b405b9c4d4d5747a6e33ed0
Task-number: QTBUG-27185# Please enter the commit message for your changes. Lines starting
Reviewed-by: Martin Jones <martin.jones@jollamobile.com>

src/quick/items/qquickflickable.cpp

@@ -2059,6 +2059,7 @@ bool QQuickFlickable::sendMouseEvent(QQuickItem *item, QMouseEvent *event)
             break;
         case QEvent::MouseButtonRelease:
             d->handleMouseReleaseEvent(mouseEvent.data());
+            stealThisEvent = d->stealMouse;
             break;
         default:
             break;