Move image info API call to media node

As other widgets may be interested in the image info, this should
be part of the DM node, which the media edit dialog then fetches
and uses to populate the MediaSizeWidget.

Other fixes:
* Remove spinner, not really necessary as the orignal size button is
  disabled if the orignal dimensions aren't set.
* Don't wait until image info call to resolve to initialise size
  widget values. These should always be available instantly from the
  attributes.

Depends on I9530ed4257e88b in core.

Change-Id: I394f49941b621059bc08c8a8f79471b2fe16c7d8
Merge "Move image info API call to media node"