Update device pixel ratio on screen change

This patch makes QWebPage listen to QWindow::screenChangedEvent
and update the device pixel ratio when it is received.

Task-number: QTBUG-36190
Change-Id: Ia28571ef241a3a2f970876f6e474b01c1ac001e1
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>