Create text widgets for System time parameters

[oliverlee]

This pull request resolves https://github.com/pydy/pydy/issues/206.

Maybe we should move the time parameter widgets so they are horizontally stacked instead of vertically? I don't know how to do that so someone else can, ideally in another pull request.

We also need to have some sort of widget or slider for fps/speedup since it's kind of dumb to tie the simulation time step to the speed of the visualization.

[oliverlee]

Screenshot when using a System object:

[moorepants]

I think you can use container widgets to make columns so that you can arrange things better.

[moorepants]

This needs to be updated with master.

[moorepants]

Switching to #310

[moorepants]

I just tried this out and noticed two things:

1. The animation is very slow. You noted that we need a speed up/slow down knob. I agree.
2. It doesn't seem possible to drag the slider to control the animation anymore.

[moorepants]

What's the status with this? I see you added some commits after my comments. Did you address them?

[oliverlee]

I noticed the same issues and I haven't looked into it yet.