Make File and Directory safely droppable

[thejpster]

Writing the shell example shows that using ? for error handling is very difficult if you have to manually clean up the File and Directory objects.

We should offer a RawFile which is an integer and File which holds a reference to the VolumeManager and hence stops you doing anything else whilst it exists.

And maybe once #79 is done, that restriction can be lifted.

[thejpster]

Looked at this in #103