refactor(at_onboarding_flutter): Add modular support to at_onboarding_flutter

[XavierChanth]

Many components and workflows in at_onboarding_flutter are not or reusable.

I want to refactor at_onboarding_flutter into 3 layers:

• Pages

  the existing option / interface

• Widgets

  the buttons and stuff currently hard coded into the pages, so parts can be embedded

• Libraries

  at_onboarding doesn't contain any of the keychain / platform dependent code
  so we must make an interface available in at_onboarding_flutter which wraps
  the behavior underlying each of the widgets

With this change there are a few other considerations I've thought about:

• Make it possible to control which widgets are included in the pages as a
  configurable list

  I've experimented with closures in the past, but they don't update when
  hot reloading flutter apps, so I'll see if I can come up with something
  static that also propogates state efficiently
  (maybe I lacked passing keys to the widgets, which is why the tree wouldn't
  update properly, it's been a while)

• Adding the APKAM widgets, with a more modular design they can be developed in
  parallel without dealing with the troubles of the nested widget code.

• I may consider starting with a blank slate, porting the old code into the new
  design piece by piece, simplifying behavior as I go.

[XavierChanth]

I chose to delay the rest of this work, extracting the file picker into a service, which is the minimum required amount of work needed by the NoPorts desktop app. I will backlog this ticket for now, and we can pull it into another sprint if we need more work done for something else.