Add a screenChanged handler slot to QWidgetWindow,
which calls markDirty() on the backing store with
the BufferInvalid and UpdateNow flags set.

Update CocoaBackingStore to create a new buffer on
window devicePixelRatio change. Use the
QCocoaWindow::devicePixelRatio() implementation instead
of a duplicate implementation in the backing store.

The plan is to replace this implementation with one
based on QUpdateWindowRequestEvent for Qt 5.4

Change-Id: I8e521c53df4ac90815613e730fe821996334721f
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>