Introduce concept of NULL plugin to retract plugins.

* NULL plugins have a shortname of L"Retracted" which excludes them
  from being listed as a plugin in Mumble >= 1.2.4
* NULL plugins are valid plugins that always return false on trylock
  so Mumble <= 1.2.3 gets behavior close to what recent Mumble has.
* NULL css, dods, tf2 and hl2dm plugins as they have been replaced by link support in the source engine.