hidden percent and max information in ProgressBar sample fixes #301

Thought this was the easiest way to fix #301. Tell me what you think.
Actually a user might want to user .percent while value is NaN which makes me think maybe we should decide what it equals at that state. 
Atm, percent this calculated through through position property (which truly is a property that has no meaning when value is NaN). But maybe we should rather set percent to just value/max.