player: add an optional mute flag when framestepping

Need to shuffle some stuff around but it's pretty simple. Mute the
audio stream before the frame step and then unmute it when we're done.
It's exactly the same as the default play mode otherwise.

DOCS/interface-changes/frame-step.txt

@@ -1 +1,2 @@
 - add optional `frames` and `flags` arguments to `frame-step` command controlling the direction and amount of frames mpv steps through
+- add optional `mute` flag to `frame-step` command which mutes the player during the duration of the frame step