Do not merge: visual compass state and transient status

Discussions in #432 and #3675 as well as frequent reports from users show that:
- the user might not know or understand that the compass has been disabled and that only the GPS will be used for direction;
- the user might forget that they disabled the compass.

This PR, opened for testing, makes the *compass disabled* state transient (instead of saving it into the preferences), and switches to a always-visible toggle on the compass activity.

Please comment on feature and suggest style changes. It might be adapted easily for the `release` branch if needed. I have reused the `attribute_compass_yes` and `attribute_compass_no` strings so as not to require new translations if we decide to add it to the release, it will be changed later for specific strings after the release.