[fmrico]

Closes #40.

New RViz tool navmap_rviz_plugin/Goal3D (shortcut h) that sets goals above the ground:

• Placing the goal: a left click puts it on the camera–cursor ray, at Distance (10 m by default). The mouse wheel changes the distance by Wheel step.
• Orientation: dragging sets the yaw on the horizontal plane; roll and pitch stay zero.
• Display: an arrow, plus a line down to z = 0 with a cross where it falls.
• Output: publishes a geometry_msgs/PoseStamped on goal_pose, as 2D Goal Pose does.

Use: used with the aerial playground (EasyNavigation/easynav_playgrounds#38) to send goals to a flying robot.

Builds: Humble, Jazzy, Kilted, Lyrical and Rolling.