Improve Attributions

[ianthetechie]

See #1379.

This PR adds a new attribution control, RichAttributionWidget as an alternative to the existing AttributionWidget (which is still valid if you want a very minimal style) and has a more expressive API built around SourceAttribution and its concrete subclasses.

• Toggle full display of non-logo attributions via the info button
• Concrete implementations for text and logo attributions
• Ability to do "hyperlink" style URI launching for all attribution types, while still enabling a completely custom onTap handler for the few that want that.

Possible discussion points:

• Logo attributions are always visible; I have yet to see a case where this isn't required in my map work but the API could be extended if desired
• I opted NOT to change the opacity of the logos as in a prototype passed around on Discord because my understanding is that this could be a big performance impact.
• In my understanding, the current attribution widget implementation is implemented in a very strange way. The default static helper doesn't even construct an attribution widget. I fixed this. I additionally think it should actually just be implemented as a named constructor rather than a static method, but open to opinions.

TODOs:

• Decide if we want any more logo attribution constructors (I just implemented the obviously necessary ones that I cared about)
• Tests (probably?)
• This project always gets a logo attribution if the control is added. There should probably be a text option instead (discussion welcome)? I also don't know where the official logo is hosted so I just grabbed it from an HTML inspection of the docs. This probably needs to change.
• Probably a version bump (not sure project policy on this)

Screenshot gallery:

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[JaffaKetchup]

Superseded/continued/replaced by #1487.

[ibrierley]

Note, I sometimes get

E/flutter (29292): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
E/flutter (29292): #0      State.setState (package:flutter/src/widgets/framework.dart:1153)
E/flutter (29292): #1      RichAttributionWidgetState.initState.<anonymous closure> (package:flutter_map/src/layer/attribution_layer/rich.dart:168)
E/flutter (29292): #2      new Future.delayed.<anonymous closure> (dart:async/future.dart:424)
E/flutter (29292): #3      Timer._createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:18)
E/flutter (29292): #4      _Timer._runTimers (dart:isolate-patch/timer_impl.dart:398)
E/flutter (29292): #5      _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:429)
E/flutter (29292): #6      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:192)

I'm currently testing the new v4 code, but I suspect this is more related to this (sorry a little out of touch atm)?

Struggling to find a good method of reproducing it though...