Add stabilized state to the stereo pilot detection signs

[jj1bdx]

• Add enum PilotState
• Use a state machine to display stereo pilot detection states
  • Stability detection is added to the previous state transition display from mono to stereo
  • Stability detection algorithm: abs(now - previous) < threshold value (set to 0.01 by heuristics)

[jj1bdx]

LGTM after preliminary tests. In some extreme cases (for example, with InterFM), 0.01 is still too small for pilot_level_threshold, but I will start with this value anyway.