Feature: separate theme option settings

[whoami-pwd]

What does this do/fix?

• Keeps settings from Theme Options Tabs in separate files
• Creates abstract classes and interface for Settings-as-Tabs and Setting-with-Tabs

NOTE Maybe we can use a better naming for that? Please let me know if you have any suggestions.

Tests

Does this have tests?

• [ ] Yes
• [ ] No, this doesn't need tests because...
• [x] No, I need help figuring out how to write the tests.

[whoami-pwd]

Looks great, @alexander-kozhukhovski . Thanks for tackling this.

@dpellenwood Thank you for suggesting and implementing this in the first place!