Import files and directories into the library.

[shibco]

As per #7, additional library organisation would include the ability to move a file or directory into the library.

Some rules for this:

1. Dropping a directory anywhere in the app should add it to the library root.
2. Dropping a file onto the editor should copy the file to the library root, encrypt it, and then open it.
3. Dropping something on top of a directory should place within that directory.
4. Ignore empty directories.
5. Ask when importing directories. Move everything but encrypt only text files. Make this clear in the confirmation.
6. Don’t freeze the UI while copying.