Add bindings for GstAudioLevelMeta

This is the first time I do such changes, so excuse me if I did something wrong.

Tests are passing after removing version check manually. Otherwise it complains that 1.19.0.1 isn't quite 1.20, which is true, but I'm not sure what to do about it with GStreamer's master not having 1.20 yet...

Also I tried to update Gir by replacing GstAudio-1.0.gir with newer version from my build, but I guess it needs to be updated in gst-gir-files repo and pulled in properly?

Fixes #340 (closed)