Add radio type and fixed mime type support and theme support

[AIIX]

Description

• Adds support for fixed mime types that Skills can send to open that specific player type ["type/video", "type/audio", "type/radio"]
• Adds support for getting theme from play() metadata and forwarding it to playback skill if available
• Adds support to display radio type

Type of PR

• Bugfix
• Feature implementation
• Refactor of code (without functional changes)
• Documentation improvements
• Test improvements

Testing

For Testing: MycroftAI/skill-npr-news#120
Requires: MycroftAI/skill-playback-control#49

CLA

• Yes

[krisgesling]

Hey, happy to add this in its current form, potentially with the theme item optional, so that any Skills that want to use it can do so and Skills still using the (track, mime) format won't break it.

What do you think?