Offline editing, create/lock sectors

[mdoering]

Description

Support the offline editing of sections of a project:

• checkout a part of the tree with all its associated data into a new dataset, creating a new sector from that new dataset to the original project so we can sync back data from there
• keep a visible lock with notes whats being done on the original tree, so noone else can checkout this part
• allow downloads for editing and subsequent uploads of modified data to the checked out dataset
• when done, resync checked out dataset to the main project and remove both lock and checkout dataset