removed 'change' event in ProgressBar fixes #390

- removed 'change' event emission in ProgressBar
- removed references to change event in doc
I think generic wording like the one  from Button "This widget does not emit any custom event." is probably better. This allows copy/paste and is probably more strictly speaking correct (i.e. as any HTMLElement mouse events for example are emitted so better speaks about no "custom" events).