Set backgroundColorProperty and keyboardHelpNode in IntroScreen

[pixelzoom]

For code review #41 ...

In MeanShareAndBalanceScreen.ts:

    const options = optionize<MeanShareAndBalanceScreenOptions, SelfOptions, ScreenOptions>()( {
      backgroundColorProperty: MeanShareAndBalanceColors.screenBackgroundColorProperty,
      keyboardHelpNode: new SliderControlsAndBasicActionsKeyboardHelpContent()
    }, providedOptions );

In most PhET sims, each screen has a unique color and unique keyboard-help content. So those options feel inappropriate in this base class. I recommend:

• Move backgroundColorProperty and keyboardHelpNode options to IntroScreenView.ts
• Rename screenBackgroundColorProperty to introScreenBackgroundColorProperty

That will put you (or the next developer) in a better position to add screens to this sim.

This also brings up the question of whether MeanShareAndBalanceScreen serves any purpose. Because if you remove those options, it's not doing anything. I would probably delete MeanShareAndBalanceScreen.ts for now, and wait to see if a base class is needed when future screens are added.

[marlitas]

MeanShareAndBalanceScreen has been deleted, and backgroundColorProperty and keyboardHelpNode options have been moved to the appropriate classes. Closing.