totem-pl-parser asynchronous interface

To fix problems with parsing playlists from slow servers in Totem, it's necessary to add asynchronous parsing functions to totem-pl-parser, so that parsing can be run in a thread. I've stuck a preliminary patch in bug #561444, but before it goes any further it would be great if other users of totem-pl-parser would take a look and see if the API's sane and suitable for their needs.

In other news, there's an interview up with our resident frockney Bastien about Totem, its past and future.