GCS: remove old WaypointEditor plugin

The PathPlanner provides its own widget and replaces
this.
PathPlanner: fix typo
PathPlanner: descript PathPlanData and capitalize it
RTH: white space changes from review

No functional changes
VtolPathFollower: optional feedforward accel compensation

When the velocity desired changes, the acceleration required to
achieve this is computed and added as a feeforward component.
This compensation is optional.