macos: Move atdec from applemedia to osxaudio

1. atdec (and possibly future atenc) could benefit from some of the code already present in osxaudio, such as audio channel mapping (see !6157 (merged)). Since it's the only audio-related piece in applemedia, it makes sense to me to move it to osxaudio instead of duplicating code needed for improvements. Will also be useful in case atenc is ever needed.

2) osxaudio plugin name is outdated, it's been a better part of a decade since they got rid of the OS X naming, people just look for mac/macOS now. It's also misleading, because elements from that plugin don't run exclusively on macOS - they also run fine on iOS. This name would make less and less sense if we start supporting other (tvOS/xrOS/watchOS etc.) Apple platforms in the future.

Having that in mind, I think it'd be a good moment to rename the whole thing to better reflect what it does/uses, hence coreaudio, because of the underlying CoreAudio library which handles all the actual audio bits done there.

osxaudiosink/osxaudiosrc were renamed to coreaudiosink/coreaudiosrc respectively, but the old names are left in place for compatibility reasons.

I went ahead and got rid of pretty much all OSX mentions in the code, so that it doesn't confuse anyone in the future when OS X becomes a completely forgotten name :) Also, this was focused on the audio parts - there's still osxvideo, but that actually only works on macOS, so it's not that bad.

Cerbero MR: cerbero!1387 (closed)

Plugin rename to be done at a later date after all.