Add function to add checksums programmatically

[krichter]

It'd be nice to be able to add checksums programmatically, e.g. as program argument, like it's possible for .torrent files. Currently, checksums can only be added one-by-one in the dialog to add torrents.

This would be useful in case deluge is not able to restore it's state, but the generated files [checksum].meta are still present in the complete or incomplete directory.