Clarify when a channel set's wheel slot contains color and when it is open

[petrvanekrobe]

A channel set of some channel functions linked to a wheel (for example Color(n)) requires a wheel slot to be linked. Add a way to more clearly indicate if the slot means "open". A wheel slot cannot be none. One way would be to add IsOpen flag to the set.

[petrvanekrobe]

TODO:

• add an enum for "open", "closed", this will preserve backwards compatibility
• document the mutual exclusivity, given by the GDTF attribute in the defining ChannelFunction, i.e.
  • Color(n) - png image (if present) or filter (if present) or color
  • Gobo(n) - png image
  • Prism(n) - prism slots
  • AnimationSystem(n) - animation wheel
  • ...

[danielmuellerma]

AnimationWheel(n) - png image MediaContent(n) - png image? ModelContent(n) - ?

[danielmuellerma]

• AnimationSystem(n) - animation wheel

png image + animation wheel