Add sailing watch face

[niclashoyer]

This is my first take at a "sailing" watch face.

Sailing races are usually started according to rule World Sailing rule 26. On the water visual and acoustic signals follow a special countdown:

• 5 minutes to start
• 4 minutes to start
• 1 minute to start
• start

This watch face is a modified countdown watch face that has 6 slots for countdowns with the following features:

• rule 26 is the default, which is 5 4 1 0 0 0. This is configurable in the settings. 0 means "start" and resets the countdown.
• beep on each signal
• pressing the alarm button "syncs" the countdown (useful if e.g. the 5 minute signal was missed to sync to the 4 minute signal)
• long pressing the light button resets the countdown
• last minute is displayed in large letters

However there are still some bugs:

• [x] periodic background tasks (a task scheduling another task) needs #105 RESOLVED
• [x] the watch face does not resign, but is unable to schedule a task in low power mode, which causes the countdown to be missed (e.g. while counting from 4 to 1 minute) is there any way around this? Maybe preventing low power mode? Resigning is not useful anyway, as having a visual countdown to start is crucial. I'm not really sure this is caused by the low power mode, but this only happens if the countdown is running for some minutes. EDIT: after some more testing, this seems to be an issue with the simulator. After investigation it seems that the simulator does not run properly in the background (e.g. if another tab in the browser is visible). See #107 .

Possible improvements:

• documentation
• add beeps for the last seconds before start
• start a "stopwatch" like timer after start