We need a decent undo and recovery system

[Tim Band]

The undo system is untested and we have experienced bugs in it. The commands it uses are complex. It moves files into a Trash folder when they are deleted, and then undoing the delete moves them back into the normal folder. During these operations the file changes its name.

The recovery system relies on this behaviour, as files are added in numerical order and deleted files are removed from that folder, so the recovery system can just add all the files still in the normal folder in order. This isn't a great solution, but it works some of the time.

When we want to add caching and streaming, all these moves and renames will cause difficulties, as we will have to make sure that the cache has to have any moved files flushed from it; and for no point.

A good recovery system with a robust undo system which doesn't move files around will give us immediate advantages and prepare the way for caching and streaming.

[Tim Band (tim-band)]

released in 0.8.0