Changes the media stuff's `@since 4.9.0` to `@since unreleased`
to reduce potential confusion on the docs. As a part of the final
4.9.0 commits, I will be changing it back to 4.9.0. We also fix the
contributing document since it was all messed up.
Move duplicated functionality to a helper function and added some docs
clarifying the behavior of the events, and fixed a few typos in
the request handlers.
Adds `GetMediaSourcesList` which returns a list of all media sources,
along with their individual media playback states. In the process I
refactored a switch which coverts the enum to a QString into its own
helper function.
