Extends MacOS MPMedia implementation to support 'seek' & 'duration'

[zackslash]

Overview:

Extends the recently merged MacOS 'MPMedia' implementation to support OS 'seek' and 'duration' display. The 'full' MacOS music widget now displays track length and allows seeking of track position.

• Supersonic responds to track position changes when the track position is changed via the OS slider
• OS slider responds to track position changes when the track position is changed via Supersonic.

Reasoning:

Feature discussed in previous PR.

Additional notes:

Extending the the initial implementation to support this was simpler than I initially anticipated, so here's the additional Pull Request.

[dweymouth]

Nice, thanks!