Refactoring backlog

[saeub]

• Consistent use/omission of const in arguments
• Prefer required keyword arguments over many positional arguments in constructors
• Doc comments
• More sane type casting in JSON deserialization
• Rethink testing without assets (e.g. reaction time task)
• Increase test coverage
• Null safety
• Replace QR package (ML Kit?)
• Remove stopwatch?
• Fix stretched loading animation on AboutListTile
• Somehow prevent passing mutable objects as data when logging (logged values must be frozen); having to remember to put .toList() every time is not very nice (see b9594ad)
• Organize imports (relative within package, sorted)
• Use ApiError.retriable to consistently show retriable/unretriable error messages
• Use final instead of var whenever possible (good reason)
• Rename subtask to trial
• Put default/initial values for stateful widgets in field definition or constructor, use initState only if access context is required