Recognize .opus as an audio format, .h264 .hevc as video formats

[OrangeChannel]

Recognizes .opus as audio (both in-application Open dialogue and Windows Explorer file definition).
Recognizes .h264 and .hevc as a video file, not a AV container.

[FichteFoll]

Will conflict with #36

[OrangeChannel]

I'll just add yours on-top of this one with you as co-author if that's okay with you. Any other formats we should add before this gets merged?

[OrangeChannel]

Also, shouldn't all demux formats supported by ffmpeg whatever aegi's ffmpeg build supports be added as well? Since going forward, it seems like ffmpeg is the only AV provider.

[FichteFoll]

I'll just add yours on-top of this one with you as co-author if that's okay with you.

Fine with me. I'm retracting #36, then.

[CoffeeFlux]

I think if we're going to keep adding various formats, it's best to only register the extensions based on the build flags set for the providers. This carries with it other benefits related to configuration, but it's a bit annoying to do with the current build system. I'll merge this once I test locally and then implement flag detection in the meson branch.