[TASK] Add Domain Model and Repository to Category

[TASK] Add Domain Model and Repository to Category

For extension developer's sake, it is convenient to provide a domain
model along with a repository. Both are relying on Extbase.

Change-Id: I7260a57dcc742f217f0ddfe19470673bc91532c9
Resolves: #38719
Releases: 6.0
Reviewed-on: http://review.typo3.org/12793
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Sebastian Michaelsen
Reviewed-by: Jochen Rau
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Markus Günther
Tested-by: Markus Günther
Reviewed-by: Ingo Pfennigstorf
Tested-by: Ingo Pfennigstorf