Implement a “Contact Trick” to update BG as watchOS complication

[callms]

As suggested by @paulplant in #479, we might consider porting the Contact Trick used by OpenGlück in xDrip4iOS.

This trick allows to update the photo of a contact, so that its profile picture matches the current BG. This, in turn, can be used on Apple Watch to provide near-realtime update of the current BG, without usual WidgetKit restrictions.

Image from OG showing how it renders on the watch:

[yurique]

I've been playing with this this weekend (mostly copying things around to add new settings to the app and "copying" things from OpenGlück) and I think I got it working:

(left icon - contact trick, middle - native G7 complication)

[paulplant]

I'll close this as an issue as we are now tracking it directly in a PR -> #505