tab_bar_item class method

I know I said that we were in a feature freeze, but this is necessary
due to the on_load changes we made.

Plus:

* Moved module class method inclusions, made possible by loading
ScreenModule in every Screen type. This is cleaner for class method
inclusion, although there's still some room for improvement.
* Added unit tests for the tab bar and tab bar items
* Moved `on_init` after `screen_setup` which makes more sense.

Ref #216 

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