Add settings for the measurement LED idle states

[dkonigsberg]

The measurement LEDs have a low-brightness "idle" state used to help the user align a target to be measured. Currently, this state is only triggered when the "detect" switch notices the device is pressed part-way closed.

A user suggestion is to make it possible for the idle illumination to be configurable in the following ways:

• Add an option for the light to come on for a longer period of time, with a configurable timeout
• Make the brightness of the idle illumination configurable (this would help with higher density transmission targets)

To support this, first a series of experiments must be conducted to determine the effect of any change here on full-brightness readings. This would be similar to the experiments conducted to track LED brightness-vs-on-time, only more specific to the variable involved in adding these settings.

[dkonigsberg]

Initial testing across a range of targets has shown that its most likely safe to allow a range of configurable brightness ranges, with the limit at 25% for transmission and 50% for reflection. The defaults for this setting should be chosen to correspond to the current brightness values.

[dkonigsberg]

This feature should now be implemented. e2b7db45620bd4a951d8dd67251a510ae67ff6fb 7e48157993cac658aef524fe1b989b851524f415 af2adc05549f2b03e18e03b3ce75b1fab45f3af6 a2a6b5217b50710b226d80393abd50ded298a557