Detect files being copied (or moved/renamed)

[nlmarco]

We should detect files being copied or moved/renamed and prevent copying them over the network.

Solution idea: We should store all of these as CopyModificaton in the database - including move/rename! Only when applying it to the sync destination, we should calculate a CopyModification+DeleteModification into one move.

The reason is that this makes handling all cases (moves AND copies) much easier and - most importantly - it makes it easier to handle remote repositories being connected to a sub-directory: For example, a "move" in the source might be a "delete" in the destination (if it is outside the connected directory).

Thus, we should persist ONLY Copy+Delete and derive all moves/renames from this.