Rename options to be consistent with other TwentyXX themes.

Rename groups to be consistent with Styles TwentyXXX plugins and WordPress nomenclature.

Rename control labels

* Remove redundant "Widget" inside widget group, "Featured" inside featured group, etc.
* Remove redundant "BG" — Styles adds "Background color" next to the label
* Change "a:before" to non-technical "Before Link"

Append 'Featured Content' options to append to 2014's default 'Featured Content' group

Warning: These changes will cause settings to not carry across upgrade if a migration script isn't written.