Mobx Updates

[brianegan]

Didn't want to override the original PR, but wanted to try my hand at making some changes to the mobx example! @pavanpodila please let me know whatcha think. I'm just learning Mobx so some (a lot?) of this may be wrong.

Updates

• Add persistence to the app using the TodosRepository
• Apply theme and Widget structures from the core to give this app a consistent look and feel with other samples
• Use localizations from the core
• Renamed a few things to keep the implementation consistent with other samples. Want peeps to focus on the Mobx differences instead of figuring out if HomePage is the same thing as HomeScreen, for example.
• Use "Model" and "Store" naming conventions from the original js version of TodoMVC -- this one is more subjective, but I liked the distinction between these two.
• Restructured the files a bit. Again, totally subjective so feedback is welcome.
• Move the Tab state out of the TodoStore, since it seemed like local "Widget State" rather than global state.

Additions

• Add Unit tests for the Store
• Add Widget tests for the HomeScreen
• Get Integration Tests running (requires adding keys to many elements of the app)

Fixes

• (Hopefully) Get CI running again -- Mobx requires Dart 2.7, but the CI was pulling down an older version of flutter with Dart 2.5

[brianegan]

Ok, took quite a bit of work but got the CI fixed. I'll go ahead and merge this example in! Thanks again for all the work :)