Fix issues on HiDPI displays

This patch fixes widgets rendering on HiDPI displays and notifies
about the a potential device pixel ratio change when the window is
moved. This addresses the blurriness when a web view is moved
between an attached LoDPI screen and HiDPI screen resulting from
the device pixel ratio not being updated.

Change-Id: I97af0199c07deac8bbe4688c665aaa6df1608ae9
Reviewed-by: Michael Bruning <michael.bruning@digia.com>