Enable drag and drop for directories

[terichadbourne]

Remaining action as documented in the comments of #100
Have temporarily just blocked dragging directories, but would be ideal to actually distinguish between files vs directories on drop and process accordingly.

related: https://stackoverflow.com/questions/25016442/how-to-distinguish-if-a-file-or-folder-is-being-dragged-prior-to-it-being-droppe