Allow Blobs to be returned from getItemImageSrc()

[arcturus]

It will be nice if we could pass blobs directly to the component for each row, and the component is smart enough to know when it has to call window.createObjectURL, revoke them, or have a policy for caching images.

[wilsonpage]

As discussed we're going to accept blobs to be returned by the user from .getItemImageSrc() and createObjectURL internally. We will cache the ObjectURL until we reach a defined memory limit, at which point we will start to revokeObjectURL to make space for new images.