Version 0.6 release for major engine overhaul

This release introduces major engine changes that shouldn't be very user visible, but should enable some nice upcoming enhancements.

Summary of changes:

• Decoupled the UI from the engine, now communicating by passing messages rather than direct mutation
• Switched from synchronous execution to async, changing some http dependencies in the process
• Sleep Track feature removed, as pandora doesn't seem to support it anymore
• Moved the station list below the 'now playing' panel
• Updated various dependencies, disabling some low-value features to try and reduce binary size a bit and improve compile times

In an upcoming release I hope to get mpris2 support for Linux, and eventually macOS and Windows media player integration as well.