get rid of LIGHT_CHANNELS, but keep channel setting for the my92xx
also clean up recent changes, channel command did not actually work

- general conversion from .ino modules into a separate .cpp files
- clean-up internal headers, place libraries into .h. guard .cpp with _SUPPORT flags 
- fix some instances of shared variables instead of public methods
- tweak build system to still build a single source file via os environment variable ESPURNA_BUILD_SINGLE_SOURCE

- add ltDimmerGPIO<index> & ltDimmerInv<index> to override / add to hardware configuration. Add pin defaults, mapped as GPIO_NONE. pwm_init() receives resulting GPIO mapping based on settings loader.
- remove bogus _light_channels variable, use this name for channel array instead (...which is vector and has size(). functions always use that value anyway)