Add non-volatile settings to GDTF definition

[wouterverlindenharman]

Many fixtures contain different "settings" that are stored inside the fixture, even when the control channel that sets the setting changes.

Some examples:

• Various dimmer curves can be selected via control channel (linear, square, ...)
• Different PanTilt speed modes can be selected via control channel (fast, smooth, ...)
• Different colour modes can be selected via control channel (raw, calibrated, high CRI, high output, ...)

The only way to describe these in GDTF today is by creating a separate DMX Mode for each setting combination, which leads to an endless quantity of modes.

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Would be easier if GDTF would contain some "variables" that can be "set" by a DMX channel, and then keep the value they were set to.

Descriptions in GDTF could then use something like an SWITCH statement to describe behavior following variable setting.

For example:

• Have a "variable" that sets the PanTilt Speed Mode: "PTSpeed"
• Using the Control channel, some slots can be defined to set the "PTSpeed" variable to different values (1=Fast, 2=Smooth, 3=Standard).
• And then on something like a Tilt channel you would have:
  SWITCH(PTSpeed):
  1: RealAcceleration="0.100000" RealFade="0.200000"
  2: RealAcceleration="0.400000" RealFade="0.600000"
  3: RealAcceleration="0.200000" RealFade="0.350000"

Software using GDTF files would then need to track the value of each variable for each fixture (and we probably should define a default startup value for each variable as well).

[petrvanekrobe]

Related to #38 and to #141 , see text here.