Feature request: Ambient mode apps

[MagneFire]

This idea was originally suggested on IRC by spectrezh.

The main concept is that it would be nice if some apps have the ability to provide an ambient mode version of itself. An example of which is the timer. In ambient mode it could still countdown while also updating the screen every second.

Things that the apps should provide to the compositor are:

• A screen update interval
• If it has support for ambient mode app (i.e. don't kill when fading to ambient mode).

[jrtberlin]

I think that this is a great idea.

WearOS uses AmbientModeSupport for this. While their implementation might not be so interesting for us we should think about adding design guidelines that are similar.

I have not found a similar feature during my first look at the WatchOS documentation. However, their human interface guidelines already suggest similar rules for active apps:

Prefer a plain black background. Avoid filling background areas with brand colors, and apply colors to specific elements instead. A solid black background reduces visual noise and improves the legibility of text.

And for a unique style, they prefer "[expression of] brand identity through font, global tint color,[...]" which sounds like a good fit for an ambient mode too.