A text progress bar is typically used to display the progress of a long running operation, providing a visual cue that processing is underway.

The ProgressBar class manages the current progress, and the format of the line is given by a number of widgets. A widget is an object that may display differently depending on the state of the progress bar. There are three types of widgets:
- a string, which always shows itself
- a ProgressBarWidget, which may return a different value every time its
update method is called
- a ProgressBarWidgetHFill, which is like ProgressBarWidget, except it
expands to fill the remaining width of the line.