Shapes may not have a unique name, but TextFrames always have. So in
order to be able to link shapes with textboxes, provide a ChainName
property that's the name of the underlying TextFrame. This kills two
birds with one stone:

- we can have a unique name for each shape
- the names can be used for TextFrame linking, as they are valid
  TextFrame names

Change-Id: Ie96f267d392d9fe5388c5eacff9b873f1639054c