SwXShape: implement reading of the RelativeHeight property

Change-Id: I479eabcd45b741a633d7d00b2bdcbd3d0dcbf0ff
SwXShape: implement reading the RelativeWidth property

Change-Id: I40d451c170699ed7616c1159a354bd93444743c0
SwXShape: implement reading the Relative*Relation properties

Change-Id: I1e99a2ff08c83ac361ab001e90047d9b8d5524fc
offapi: document the 4 new properties which are no longer read-only

Change-Id: Ia2844af91159bf4a512fcb70883fc6a1eddfce35