Create BranchHandle

Scattered along various wicket Plugins in community and wpm code, the same logic is repeated over and over again. This is complex logic and needs to be contained in one place ( DRY ).

Create an interface called BranchHandle and it implementation. That class encapsulates the business logic to get document variants from the handle or from version history and additional information that cannot be retrieved from the workflow. The workflow explicitly deals with variants under the handle.

See commit messages for more detailed information on what and how.