- disco stone:
- rename to st_disco, st_disco_light, st_disco_medium, st_disco_dark
- state attribute with values LIGHT, MEDIUM, DARK
- message "lighten", "darken" change by one degree
- message "signal" turn on to LIGHT on value 1, off to DARK on 0
- no recursive disco messages on new API - use named groups
- backward compatible recursive disco messages on old API
- maintain object identity on switches
- fix crashes on stoneimpulse on dynamite explosions
- experiment: blue puzzle explosions equal yellow ones