Blender Git Loki

Anim UI: Playback Sync menu improvement

Relabel the Playback Synchronisation menu so that it's clear:
- what does happen (instead of describing what does //not// happen), and
- that the synchronisation options don't just affect audio.

The changes are:
- Change label from "Audio" to "Sync"
- Change the labels of the sync enum:
- No Sync ? Play Every Frame
- Frame Dropping stays the same
- AV-sync ? Sync to Audio

The "Audio" label has moved one option down, as that option does
actually relate to audio.

Reviewed By: looch, Severin, HooglyBoogly, campbellbarton

Differential Revision: https://developer.blender.org/D9269