Clean up Performance Tracking plugin settings code

[mykola]

The settings page for the plugin has been added using code structure that doesn’t match the rest of the plugin. Refactor the code using the WordPress best practices and organize it to match the rest of the plugin code.

Tasks

• [x] Move settings into class
• [x] Implement unit tests
• [x] Extract strings into constants
• [x] Remove code duplicates
• [x] Enhance code

Current edition doesn't include test for separate classes - it's just idea how to split main code and approve that it works as previously with old tests.