Add flag to debug observers
Add timeline options for onion skin, loop section, and animation direction

- Add red/blue onion skin type
- Add app::calculate_next_frame() function to calculate the next frame
  depending of the timeline configuration (loop range, animation direction)
- Add app::finder() to simplify the access to widgets loaded from xml files
Change ConfigureTimelinePopup layout
Change default onionskin opacity step setting
Fix Onionskin_Merge type