- commands do now have to start with @

- commands do now have to start with @
- add commands @size, @material, @light
- make "BlackSign" texture default
- make @B, @R, @L, @Y open close their frames automatically (this can be
  avoided by setting the @material manually)
- add number variants for those 4 sign commands: @Y2, @B5, etc (according
  to the spec; defaulting to the respective biggest panel size, i.e. @B = @B3)
(detailed description will be added to $FG_ROOT/Docs/)