Illumination LED control range limit.

[easytarget]

Inspired by the welcome recent PR's and comments from @ericfont ..

Reconfig the LED illumination lamp control:

• Pin: a valid GPIO number or undefined for no LED.
• MaxVal: PWM value for full on;(100%); allows max value to be restricted for safety/convenience etc. default 1024.
• MinVal: PWM value for full off (0%); Usually 0, but allows a value above this to be set, preventing the light being fully deactivated; which is another use-case I want to support.
• Special case: Min=Max; lock lamp to that PWM value permanently and do not display slider in UI
• Cater for Inverse Operation: Support for reversed drive LED's (eg PNP vs NPN) by allowing MaxVal to be lower than MinVal; make sure all the maths is handles this properly, and test..
• Add notes to these config options to warn of overheating, image glitches and the need for a righteous PSU.

[easytarget]

@ericfont Just a quick note that this issue and the associated PR's are not off my radar, but I'm head-deep in some python stuff at the moment and will come back to this once I have more head-space.. and time on my hands. They are all good sugestions but I just need to get them properly merged and documented etc.. and that will take more time than I currently have free for esp-cam stuff.