Workflow to support a checkout of a branch version

Support for a new workflow action to checkout a branch version (by label). A checkout means that the Preview below the handle is replaced with the versioned node. Conditionally, the Preview first gets versioned if it has any changes compared to the version history of the label for the Preview. Assume in the diagram above that the Preview has label L1-preview, and that L2-preview gets checked out, then before the Preview gets replaced by L2-preview, we need to check-in the Preview if it has any changes compared to 1.8. If not it can just be replaced, otherwise a 1.10 version is created and the L1-preview label pointer moves to 1.10.