Clean up Drupal.panels_ipe.init()

While working on #2657086: Use Drupal.t to translate all strings in IPE Backbone Views → Drupal.panels_ipe.init() to be rathe messy so I spent some time refactoring it. The result is a self documenting method with self documenting method calls.

This patch is based on the work in #2657086: Use Drupal.t to translate all strings in IPE Backbone Views → and is therefore postponed until that issue lands.