versioning

[magwas]

Object versioning should be implemented. Both on server and client side. See #14, #15 Ready when:

• version management primitives are working (opening a new version, committing it)
• queries give back the correct version of an object
• the gui is able to handle versioning, esp. objects with a newer version existing.
• all things unit tested Not scope for this bug:
• merges, pull requests