Support branch information on the document variant

Support a new mixin or property for hippo:document to indicate to which label (branch) the Preview/Live variant belongs (even if we can support linear version history per branch, this is more efficient when loading the variant in the HST because the HST might be able to skip version history if the label on the variant is already correct)

Suggestion to add following mixin:

[hippo:branchInfo]  mixin
 - hippo:branchId
 -hippo:branchName

Via (community) workflow it must be possible to set the branch on a document variant. Most likely we can best call it 'branch' even though in version history we support it by using JCR label on versioned nodes. The effect we aim at is to support multiple branches of a document.