[Channel Manager] side-by-side editing checkout the right document branch

Channel mngr side-by-side editing can be supported in case of projects to automatically checkout the correct document branch. For example when looking at the Christmas edition in the channel mgr, then when editing content in the Channel mgr, we can automatically checkout the Christmas edition (if that is not currently the preview variant). Of course this is only possible when the document is not being locked by someone else.

How to create an editable instance for a document depends how the concept of branches is implemented. Therefore add an interface and default implementation for obtaining an editable instance.