Create a joint_trajectory_controller rqt plugin.

Initial plugin features:
- Allows to select any _running_ joint trajectory controller from any active
  controller manager.

- Two modes:
  - Monitor: Joint display shows actual positions of controller joints
  - Control: Joint display sends controller commands

- Max joint speed is read from the URDF, but can be scaled down for safety.
Merge pull request #103 from pal-robotics/rqt-jtc

joint_trajectory_controller rqt plugin