Add missing sectionIndexTitles method for tableviews

[jdanthinne]

Checklist

• [x] All tests are passed.
• [ ] Added tests.
• [x] Documented the code using Xcode markup.
• [x] Searched existing pull requests for ensure not duplicated.

Description

I've added the missing sectionIndexTitles method.

Motivation and Context

I needed sectionIndexTitles to be displayed, but the method was not overridable, so I've added it.

Impact on Existing Code

None.