Preferences view needs cleanup

[pixelzoom]

Noted while working on https://github.com/phetsims/number-suite-common/issues/35...

NumberSuiteCommonPreferencesNode, NumberPlayPreferencesNode, and NumberComparePreferencesNode all need major cleanup. There are multiple controls being created in the constructor, and they are not properly encapsulated. This will be trouble for future maintenances and PhET-iO instrumentation.

Self assigning.

[pixelzoom]

The above commits will look like major changes. But what I did (in a nutshell) was to factor out a class for each control that appears in the Preferences dialog. So we now have these classes, whose names match their labels:

• SecondLanguageControl
• ShowOnesControl
• SubitizeTimeControl

@chrisklus please review. Close if OK.

[chrisklus]

Thanks @pixelzoom, I really like these changes. I tested all the enabled-ness combinations and control functions. Closing.