Improve axclient.py

[Torchtopher]

As someone who uses axclient fairly regularly, there were quite a few limitations that made it somewhat annoying to use. This pull request improves the functionality of axclient adding:

• Support for multiple actions within the same window
• Loads all actionlib servers without having to type the topic name
• Has message file for viewing when sending actions, useful for enums/comments in the raw message file

Here is what the GUI looks like now.

Now run with just rosrun actionlib_tools axclient.py and the rest is taken care of.
This is my first contribution but I wanted to share the changes I had made. Would be happy to make any changes/improvements needed to merge!

[130s]

Not a maintainer, just reviewing voluntarily.

• Support for multiple actions within the same window

What this means is unclear to me. Does the gui with your change 1) take in multiple different goals, 2) show feedback and result per each, on a single window? That sounds great (while I personally can't think of a usecase), but also makes me worried if the gui could get confusing. Would you mind clarifying?

[Torchtopher]

@130s Yes, it does allow you to send multiple goals and see the feedback for them. The gui is still relatively simple, as you only see the information for a single action at a time and use the Action Selector menu at the top to switch between them. The usecase, at least for us, is that often you need to call multiple actionlib servers, or don't quite know which one you want, and can then look at an overview of all of them in that menu. Let me know if you have any other questions!

[130s]

@mjcarroll @jacobperron You're listed as maintainers IINM. Is this something that can be merged into ros1 branch? I'm inclined to say this includes new features.

Separately from this PR, I'm interested in ros2-fying axclient.py (discussed in ros2/rcl#1079). If there's a chance of this PR getting merged into ros1 then I'll wait for the code to be finalized (to avoid having to repeatedly cherry-pick).

[Torchtopher]

@mjcarroll @jacobperron Any update on this? I would be happy to make any changes needed to get this merged.

[mjcarroll]

@Torchtopher great seeing you at FIRST championship. I will add this to my list of things to review this week.