ProgressBar Conditional remove_when_done

[kenodegard]

Inspired by the discussion in #1025 (comment).

Instead of using a threaded approach for remove_when_done processing we check for removals as part of the rendering process instead.

@jonathanslenders this feels like a hack at this point, not sure if we'd want the get_progress_bar/set_progress_bar/get_counter/set_counter to be extending the AppSession or whether using this separate ProgressBarSession is preferred.

If we continue down this contexvar path then maybe AppSession should be made more generic to where anything can be set in a session? So we'd introduce something like get_contextvar(key) and set_contextvar(key, value). Just thinking out loud here.

[kenodegard]

Pinging this.

Been using this change for the last week and haven't run into any issues. So I don't have any concerns with implementing remove_when_done as a Conditional.

The only open question is whether the get_progress_bar and get_counter are implemented in the desired way.

[kenodegard]

@jonathanslenders let me know if this is of interest or whether I should just close this and implement a monkey patch on my end - thanks