movement: Add "instant" led_duration option.

[WesleyAC]

This illuminates the LED only for the time that the button is pressed, and turns it off as soon as it's released, which is the behaviour of the original watch.

I chose a led_duration of zero to represent "instant" and all bits set to represent "no LED", which is arbitrary but seemed more sensible to me.

This PR also makes this the default. This is a matter of personal preference, but "instant" is the behaviour of the original watch, and seems like the thing more people would expect. Feel free not to take that commit if you disagree, though.

[WesleyAC]

Not tested on the physical watch yet, just the emulator. Will try it on the physical watch whenever I bother to take it apart and reflash it next.

Here's what the settings screen looks like:

[WesleyAC]

Tested on hardware and seems to be working well

[814d3]

Very nice, I will test it and report back after 1 to 2 weeks. edit: Short Feedback: Looks good to me and is running well. Testet it (green board) also within timer_face. In interval_face it is not possible to use the LED because of the button assignment. A short light button press does nothing (LED is activated shortly); a long press enables settings mode of the face. From my point of view this should be swapped, but that's not a problem of this change/pull request. edit2: In addition, it's the same behavior with alarm_face, where a long light button press enables settings mode as well. The behavior is better with thermistor_readout_face, as a long light button press enables the LED, but this long press is also interpreted as a short press and the face displays the log time instead of the temperature. In the simple_clock_face I like the instant light, but within the other faces it's a bit odd. Just my two cents to think about. edit3 after one week: Nothing more to add. The instant light feature works as expected except for the above mentioned faces, that I tested. I'll stick with it, as I mostly need the LED while the simple_clock face is active.

[chris-v8]

My solution suggestion would be to merge both instant, and timed LED. In simple face, only the instant LED would be active. In every other face, LED could be 5, 3, n seconds. This would solve the conflict of of faces using short and long press. Although I wonder if making additional if statement would have a drawback on the CPU?

[wryun]

To take the LED in the other direction (more light!), I raised https://github.com/joeycastillo/Sensor-Watch/pull/394/files

:)