Enhancement - Volume Control

[GeoffTearle]

Detailed Description

AsteroidOS supports controlling the volume of the currently connected phone.
This may not be currently implemented in Starfish, so as part of this issue, investigate whether this is already working and, if not, implement it.

The scope of this should be limited to native music players at this stage, and android music players looked into in a follow up issue.

Context

Feature parity! We should strive to get Starfish to feature parity with Telescope (and one day the Android client).

Possible Implementation

The implementation of the above should be quite simple as I believe that both of these features are supported by libwatchfish, so it should be as simple as using the functions provided.

[robthebold]

@GeoffTearle Are you referring to the volume slider on the Music applet on the watch?

[GeoffTearle]

@robthebold Yes that's the one!

Sorry if the way I created these issues is a bit confusing, I made them in from the perspective of the various subsystems of the synchronization service.
Where for users it probably makes more sense to think about it from a watch applet perspective.