qtFred QOL: Campaign Editor UX

The campaign editor has some UX issues.

- [ ] To improve workflow an arrow button to add missions to the flow area, like in the reinforcement editor, would be intuitive
- [ ] The area to start dragging the arrows should include the semicircle, as well as be a little larger on the pill as well
- [ ] Making an arrow connection to a pill should be more lenient
- [ ] Repeat sexps should automatically be last, regardless of when they are added so that modders don't have to remember to do this on every single mission.