progress: Improve the Progress Plugin

[RobLoach]

This change improves the progress plugin by...

• Allows reseting the progress bar to allow running another progress bar after the first one is done
• Replaces the message whitespace so that there's no overlap in messages
• Scopes the variables within the function
• Renames the global variable so that it's namespaced a bit

Here's a good way to test it...

progress 0 "Doing something cool!" && progress 40 "Hello?" && progress 60 "Almost done" && progress 100 "Close" && delay

[akinomyoga]

The message Done! seems to have been dropped. Is this an intentional behavioral change? What is the rationale?

[RobLoach]

It's kind of clear when it's done, because the application continues?

[akinomyoga]

That explains that the new behavior works fine, but it is the same for the previous behavior. Rather, the previous behavior was even more clear about that. Also, does that "?" mean you don't remember why you did it? So, can I think it was not the intentional behavioral change? I avoid breaking changes unless there is a clear reason. Could you recover the original behavior?

[RobLoach]

When comparing other progress bar approaches, like that in apt, I found that it didn't present a Done! For each progress bar that was displayed. Tried to mimic that behavior.

The removal was intentional, but I'm happy to restore it.

[akinomyoga]

Another thing that this PR changes without explanation is the accelerating growth of the progress bar. Now it is linear. What is the reason for this change? Did you remove that knowing the original intent? Or did you just remove it because you don't understand it?