Causes the given text to appear next to the progress bar.
If text is null and show_text
is true, the current value of fraction
will be displayed as a percentage.
If text is non-null and
show_text is true, the text will be displayed. In this case, it will not display the progress
percentage. If text is the empty string, the progress bar will still be styled and sized suitably for containing text, as
long as show_text is true.
| this | |
| text |
a UTF-8 string, or null |