Backlight brightness set with PWM

[charlespax]

In Electronics version 0.10 we should be putting LCD_BL on a PWM pin, so the brightness can be adjusted. This issues addresses the back end implementation. The minimum implementation is to have the PWM level hard coded as something like #define LCD_PWM in t400.h.

For the user interface implementation see #141.

Electronics issue: PaxInstruments/t400-electronics#109

[charlespax]

This has hardware implications, so I'm moving it to Firmware version 0.2, so we can address it now.

[cibomahto]

It's on PD7, which is a PWM:

T0/OC.4D/ADC10 (Timer/Counter0 Clock Input or Timer 4 Output Compare D / PWM output or Analog to Digital Converter channel 10)

Timer 4 is being used by the button polling routine at the moment, but it might be possible to rearrange these.

[charlespax]

This is not essential to the graphing theme. It's a nice-to-have feature. Moving to the UX milestone.