Mini-widgets: Add low voltage warning for Battery Indicator

[ArturoManzoli]

• Adds a configurable "Low Voltage" warning that triggers 5 seconds after the voltage drops below setpoint.

[ES-Alexander]

@ArturoManzoli This functionality is already built into the autopilot, as a failsafe - it seems problematic to replicate it here without integrating with the autopilot's version.

I'd recommend either:

1. allowing the control interface here to integrate with the relevant autopilot parameters (e.g. reading them on startup and allowing changing them), or
2. linking to the BlueOS failsafes page (or if necessary the autopilot parameter docs) to encourage users to configure them themselves