Allow picking of directories.

This is used e.g. by k-9 to select an attachment storage path.

There are a number of intent filter conventions for picking directories.
In k-9's code I found the following:

- action org.openintents.action.PICK_DIRECTORY, scheme file
- action com.estrongs.action.PICK_DIRECTORY, scheme file
- action Intent.ACTION_PICK, scheme folder
- action com.androidworkz.action.PICK_DIRECTORY, scheme file

Implemented is the third variant, as it's the most generic way to
describe the intention.