[enhancement] Add gcode_macro variable as input value

[viappidu]

Believe is exactly what @TodWulff mentioned in #24 Shouldn't be too hard to replace the Analogpin layer for a variable one. Is it?

Just in case a tip is needed, the variable name should be split if not first level, and, well, updated by the effect. My use case... [gcode_macro _CLIENT_VARIABLE].hw.leds.caselight.brightness (In this example the variable name for the effect would be '_CLIENT_VARIABLE.hw.leds.caselight.brightness')

[julianschill]

I plan an effect where you can set the value with a gcode command. So you would call SET_LED_EFFECT EFFECT=gcode_effect VALUE=42 and this would change the value of the effect similar to analogpin. Maybe I will add some effects, that change the speed based on the value, lets see. For now only one Value per effect (so no different changeable values for different layers)