Feature: Allow changing text of SpIndeterminatedProgressBarState

[JanBliznicenko]

SpIndeterminatedProgressBarState has fixed text 'Processing...' above the slider. In many occasions, that is not exactly the desired text ('Loading', 'Saving' or whtever might be more appropriate). Right now, the class has to be subclassed for own text there. It would be nice if the text could be changed after it was opened (for example first show 'Downloading', then 'Unpacking')