Change ve.dm.DocumentSlice to a mixin to ve.dm.LinearData

Document slice only ever contained linear data, with extra functionality
to preserve the range. It pre-dated LinearData, but now we should
refactor it to reflect its purpose.

Change-Id: Ifc908f7526c83a43a51372c8d2494d7260e7facd
Merge "Change ve.dm.DocumentSlice to a mixin to ve.dm.LinearData"