ResourceWatcher: Send the values as QVariants not QDBusVariants

I have no idea why we chose to send them as QDBusVariants, but it causes
problems in extracting the data at the other end.

Also pass resources as strings, and not QUrls. DBus does not natively
support QUrls and passes them as -

struct { string nepomuk:/res/.. }

instead of just 'string nepomuk:/res/..'