[backtrack_behaviour] This implements the backtrack recovery as an action server

This implements the backtrack recovery as an action server in a separate package. Maybe it should eventually be moved to scitos_2d_navigation but I'm leaving it here for now. One problem is that it is preempted when close to waypoints but this doesn't seem to lead to any other issues than that the recovery is not executed. If it goes further away from the start than the requested meter's back by planning some odd path, the action gets aborted. Tested this on Rosie but it is hard to see any gains in scenarios that are not staged by me.