browser-plugin: Add more OGG mime types

shared-mime-info identifies ogg media as audio/x-vorbis+ogg and
video/x-theora+ogg (easy to see in Nautilus).

I haven't seen any websites serving these MIME types, but when
loading media from local disk (quite common for OLPC shipped content)
these MIME types do come into play.

Add them to the list so that the browser plugin continues to be used
to play such media.