Mission assistant

Working toward fixing #953, by adding a dialog before uploading a mission that queries the user if he wants to finish the mission with a RTL point (and a takeoff if necessary).

**WIP**  - As the dialog should say "skip" instead of cancel.

![screenshot_2014-10-06-13-46-36](https://cloud.githubusercontent.com/assets/3289118/4533168/f2fef6c2-4d99-11e4-88b5-6a1130a90443.png)