Graphical conflict resolution

[saberraz]

It would be good to have some tools (maybe processing algorithms) to:

• Compare 2 versions of a the project (only survey layer)
• Generate diff files: updated, deleted, added
• Apply changeset between 2 versions

[PeterPetrik]

Also it would be nice tto have 2 modes for changes, apply all in diff, or apply/reject one-by-one

[alexbruy]

Related pull-request #397.