This PR changes the way how stop areas and elements are discovered.
The elements of stop areas are now automatically loaded when becoming visible in the camera view port.
Additionally stop areas are no longer displayed as circles. Instead there is an overview mode that displays a dot per stop area and colors it to represent whether it is unloaded (gray), loaded and complete (green) or loaded and incomplete (red).

To achieve the desired goal some long planned re-factoring had to be done. That's why this is such an irresponsible massive PR.

Major improvements are:

• better performance due to the movement of basically all business logic code to a separate thread
• improved code separation especially for widgets/screens which previously had to contain some logic because providers couldn't access each other

Minor ToDos/Bugs:

• dependent elements are not re-evaluated when an element is updated/uploaded
• stop area state is not updated on element update/upload
• "expert questions" preference is currently ignored (set to false)
• revisiting onboarding leads to "stream error"
• improve "completed marker" show animation and or style - change later
• upstream cluster bug: SuperclusterMutable.insert() and SuperclusterMutable.load() produce different results rorystephenson/supercluster_dart#5

Other changes:

• websites from about or help screens do now open in the app window (can be reverted if desireable)
• stop area query window has been reduced