Sets whether the progress bar will show a text in addition to the bar itself.
The shown text is either the value of the [[email protected]:text] property or, if that is null, the [[email protected]:fraction] value, as a percentage.
To make a progress bar that is styled and sized suitably for showing text (even if the actual text is blank), set [ [email protected]:show-text] to true and [[email protected]:text] to the empty string (not null).