Added watch_disable_display

[voloved]

The code states to disable the slcd when not in use, but that's not possible with the current code.
This addition allows the LCD to be disabled more easily.

Tested in the simulator and hardware.

  *          about 4 microamperes with all segments on. There is also a slight power impact associated
  *          with updating the screen (about 1 microampere to update at 1 Hz). For the absolute lowest
  *          power operation, update the display only when its contents have changed, and disable the
  *          SLCD peripheral when the screen is not in use.