Add mk3d to video extensions and enable double cache

Adds .mk3d to video extensions and enables double cache for its MIME type like it's done already for video/x-matroska (.mkv).

mk3d is an official extension [1] supposed to be used for 3D videos. It is also the default one you get from tools like MKVtoolnix when you set the stereoscopic mode.

Adding it in advancedsettings.xml is not enough, as double cache won't enabled for it, while it is for "regular" mkv files. They're no different, in fact you can rename a .mk3d to .mkv - it's just a specific file extension for 3D files.

Surely safe enough for Helix.

[1] http://matroska.org/technical/specs/notes.html#MIME