Start app (#2927)
* WIP
* Apply default options when screen options are null
* modal animations
* fix tests
* refactor
* refactor
* refactor
* refactor
* test fix
* topBar anim
* child disappear bug
* Update app.js
* Update WelcomeScreen.js
* fix tests
* bottom tabs
* stack id
* fix test
* refactoring
* merge fix
* Update app.js
* merge
* comment to fix test
* merge options
* start app anim
* anim