Add Secret changeset group to the Synchronize view

To follow up on work done for #614 (closed), I would like to add a Secret changeset group to the Synchronize view. As the name suggests, this new group will show all of the changesets in secret phase contained in the local repository. The Secret changeset group will appear alongside the existing Incoming, Outgoing, and Uncommitted groups.

Additionally, a command will be added to the context menu to enable shifting the secret changesets into draft phase. This mirrors the existing command to shift outgoing changesets into secret phase.