pbutils: Expose more information about media types (audio, video, container, ...) in missing plugin messages

Submitted by Jonathan Matthew

Link to original bug (#549111)

Description

It would be useful to be able to inspect missing-plugin messages, so that applications can avoid requesting installation of plugins that they won't use anyway.

The only use case I have is for audio-only applications that frequently have to deal with video and random other files. Currently, when rhythmbox runs across a video file for which no decoder is installed, it will request installation of the decoder, then, after forcing the user to hit a few buttons, it will ignore the file, because it's only interested in audio files.

What I'd like to do here is ignore the file immediately, by checking the missing-plugin messages to see if a video decoder is required.

I'm not really sure what an API for this would look like, or what information it would expose. The code I'm playing with now will get the type and (if it's a decoder) caps out of the missing-plugins message and then check media types. This doesn't really feel like it's guaranteed to be stable, and checking media types never seems like a good idea.