New SizedAsset domain model

as suggested in https://github.com/mgoldbeck/TYPO3.Media/issues/2,
this introduces a new domain model class SizedAsset
which can signalize Assets having a "size", just
like it's the case for images and videos.

The present Models Image, ImageVariant and Video
are adjusted accordingly, as the persistence migrations
are.

Change-Id: I4c0d4ea2c83daaec98f5bc5d352ed854b8b864e6