rqt_joint_trajectory_controller refurbishment

[christophfroehlich]

Background

rqt_jtc should be fully functional since #890 again, but it could see some refurbishment.

I copied the todos from rqt_jtc to this issue to make it more visible.

@Noel215 wrote this when porting this to ROS 2 with #356, I leave this uncommented here and open for discussion.

• Better UI support for continuous joints (see DoubleEditor TODO)
• Can we load controller joints faster?, it's currently pretty slow
• If URDF is reloaded, allow to reset the whole plugin?
• Allow to configure:
  • URDF location
  • Command publishing and state update frequency
  • Controller manager and jtc monitor frequency
  • Min trajectory duration
• Fail gracefully when the URDF or some other requisite is not set
• Could users confuse the enable/disable button with controller start/stop
  (in a controller manager sense)?
• Better decoupling between model and view
• Tab order is wrong. For the record, this did not work:
  QWidget.setTabOrder(self._widget.controller_group,
  self._widget.joint_group)
  QWidget.setTabOrder(self._widget.joint_group,
  self._widget.speed

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