letterfall/numberfall: Start Letterfall 2: Numberfall.
letterfall/font.sscm: Add :*!. to font.

  Needed for Numberfall, particularly title "Letterfall 2: Numberfall",
    to indicate dead letters, and to punctuate help screen.
letterfall/kgtk.sscm: Add support for backspace and return key.
letterfall/numberfall.sscm: Create title screen.
letterfall/numberfall.sscm: Fill help screen.