Also
- rename SetOfByte to SdrLayerIDSet
- add asserts in SdrLayerAdmin::GetUniqueLayerID so that we don't
  allocate overlapping SdrLayerID values
- add a new constant SDRLAYERPOS_NOTFOUND to be returned from
  SdrLayerAdmin::GetLayerPos

Change-Id: I3bb3489f9338e3d02c4040bcbd811744699941c8
Reviewed-on: https://gerrit.libreoffice.org/37467Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>