Add light sensor event that only fires when valid reading is available.

[GeorgeWort]

New event MICROBIT_DISPLAY_EVT_LIGHT_SENSE_READY is introduced to resolve two issues.

• Invalid results in first 3/4 readings.
• Invalid results when using bluetooth.
• Invalid results when touching the pins.

This allows for four different behaviours depending on how you interact with the light sensor as shown in the code sample below.

#include "MicroBit.h"

MicroBit uBit;
int lightLevel;

bool valid_only = false;
bool use_new_event = false;

void lightLevelReady(MicroBitEvent) {
  lightLevel = uBit.display.readLightLevel(valid_only);
  uBit.serial.send(lightLevel);
  uBit.serial.send("\r\n");
}

int main() {
  uBit.init();
  if (use_new_event) {
      uBit.messageBus.listen(MICROBIT_ID_LIGHT_SENSOR, MICROBIT_DISPLAY_EVT_LIGHT_SENSE_READY, lightLevelReady);
  }
  else {
      uBit.messageBus.listen(MICROBIT_ID_DISPLAY, MICROBIT_DISPLAY_EVT_LIGHT_SENSE, lightLevelReady);
    }
  uBit.display.readLightLevel();
  while (1) {
    fiber_sleep(20);
  }
}

• Using the old event and passing either nothing or false to readLightLevel gives the same behaviour as before, simply returning the values (including invalid ones) every time a reading is taken.
• Using the old event and passing true to 'readLightLevel' causes an error value of -1 to be returned for the first few readings, and stale but valid readings to be returned whenever invalid results are produced.
• Using the new event means that readLightLevel is only called when a valid reading is available.
  • Passing true to readLightLevel when using the new event means that even if a bad result is produced between the event firing and the call to readLightLevel, a valid reading will still be returned (either the reading taken when the event was fired, or a more recent valid reading).