Use strings not enums for directory types.

This allows plugin modules to define their own directory types
while still using the @DirectoryType qualifier, rather than having
to define a new qualifier.

Change-Id: I4809c94303a10a9e412615be4f8d10a086ace92b
Reviewed-on: http://gerrit.dmdirc.com/3176
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>