Implements TemplateDropdown class to create custom template dropdowns

What does this MR do?

Refactorize template dropdowns. This MR creates a base TemplateSelector class so it can be reused for multiple types of templates.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together