Add an about (and a settings stub) dialog

- activated via search menu (tap on search)
- remove stack view (features not used) from main screen
- simplify state handling
- remove condensed mode from placemark dialog, still too confusing
- remove re-anchoring of routing button (keep it simple, stupid)
- add dependency on qt quick controls 2 (slide view)
- remove close button from dev dialog in favor of back button
- simplify navigation manager: guidance mode == visible
- konqi icons from https://community.kde.org/Promo/Material/Mascots

(cherry picked from commit b3f9d543)