[release] version bump to 13.1 rc1
Merge pull request #4742 from MartijnKaijser/13.1rc1

13.1rc1
[DualAudio] Update README info
[DualAudio] make Audio Engine Interfaces support 2nd audio
[DualAudio] make Audio Engine Utilities support 2nd audio
[DualAudio] make Audio Engine ffmpeg encoder support 2nd audio
[DualAudio] implement ActiveAE code for 2nd audio
[DualAudio] create 2nd Audio Engine instance for AEFactory
[DualAudio] add detection of NULL audio sink for failsafe
[DualAudio] add 2nd audio settings to xml configuration
[DualAudio] implement code of 2nd audio for GUI settings
[DualAudio] implement 2nd output of GUI sounds
[DualAudio] dvdplayer: 2nd codec and stream manipulation
[DualAudio] dvdplayer: add 2nd dvdaudio
[DualAudio] dvdplayer: implement code logic for 2nd dvdaudio playback
[DualAudio] dvdplayer: sync for 2nd dvdaudio playback