Move duplicates to a different directory

New feature: fclones move <target_dir> moves duplicates to a given directory.

The directory structure is preserved, so files with same names don't conflict with each other and it is easy to undo the move manually.
If files are moved within the same mount-point, only file metadata is modified, which is very fast.
If files are moved to a different mount-point, the data are copied first and the source files are removed afterwards.