Hardened calling plugin frame drawing methods

  - Added try: except: around calling plugin .draw_frame()
     and .draw_splash()
  - In case of failure, draw a default message - currently
     a red screen of death

  - Also change the menu scrolling code to draw a bar, not
     the arrows as previously
Add brightness control to Disco Floor plugin

  - D-pad up and down now controls the brightness (10 steps)
     and d-pad left and right regenerate a new set of colours
     (useful when the pattern is stopped (0 FPS)