Remove Dragula fork

The new channel manager uses a fork of the Dragula library: https://github.com/onehippo/dragula/tree/hippo. This fork includes the 'direction as function' feature (pull request #336) so the drag direction of containers can be specified per container.

Recent comments in pull request #336 suggest the direction can also be changed in a different way. Is that is true we could get rid of our Dragula fork, which would be good.