Make the image model persistent

Make sure the image model can switch parameters when the image
source is switched, rather than having to instantiate a new
image model.

Also, when the image is changed, retain aspect ratio but make sure
to preserve the width of the previous image (unless the new image
is smaller)

Added 'resizeToBoundingBox' and 'scaleToThumbnailSize' static methods
in MWImageNode that return scaled dimensions based on the image
dimensions and the bounding box detals.

Depends on core change Idbb8b29db8e

Bug: 69450
Bug: 68000
Change-Id: I4327c86a9127f46da5522dc1197411fda757d5a4
Merge "Make the image model persistent"