Add guest mode

[abrain]

Add a mode that temporarily replaces the idle screen with a static background (ideally an image), a logo, and a short text. This mode is meant to provide a neutral look when you have guests and want to conceal data that is usually shown on the screens. Or just because it looks nice. Alerts will be shown as usual.

[abrain]

As an interim solution, one can use the image component introduced in #30. It has the drawback, that the other components have to be removed and added again later.