Backport MPF 0.80 Updates to (dev) 0.57

This PR backports some improvements from the 0.80 branch into dev for release in MPF 0.57.

BCP Machine Vars and Settings

This PR fixes a bug where the machine variables were sent to a BCP client repeatedly, due to an iteration mistake. It also improves the schema for how settings are transported over BCP to better group them by setting type.

Attract Mode Carousel

This PR changes the default Attract mode to inherit from Carousel, enabling an attract carousel without the addition of a dedicated mode. Attract mode includes custom code for triggering a game start, so users can't simply extend Attract from Carousel without losing that functionality. With this PR, the default Attract mode handles both. If no carousel is provided in the mode_settings:, the carousel functionality is ignored.

FAST Audio Board

This PR improves the serial handling and persistence of FAST Audio board volume levels by exposing them as machine variables.

Shot Group Events

This PR extends the shot_group hit event to include the shot that was hit, for better control of events triggered by shot group hits.

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missionpinball / mpf