Moveit: setup_assistant panel for joint_limits.yaml

[v4hn]

The joint_limits.yaml file is generated by the setup assistant, but there is no way to edit the file from the assistant.

It would simplify setting limits if we could have sliders for optional position limits that move around the robot visualization too (just as for adding named states).
Joint velocity / acceleration limits could be simple edit-fields with checkboxes to enable/disable them.

To make this useful in practice, it should also be able to load the current joint_limits.yaml if there is one already.

[MohmadAyman]

I could do that, after we get #1243 merged.