Provide a function to draw progress bars on the dock icon

QtMacExtras should provide a function to draw progress bars on the dock icon. Both Windows 7/8 and Ubuntu Unity provide APIs for drawing progress bars, the former of which is currently being implemented in QtWinExtras.

Unfortunately OS X has no native API for doing so, and many OS X applications draw their own custom interface elements for this, all wildly different.

Here are some images of the various progress bars used by applications: https://bugzilla.mozilla.org/show_bug.cgi?id=548763

I'd prefer attempting to mimick this: https://bug548763.bugzilla.mozilla.org/attachment.cgi?id=549506, since it's the closest to an OS provided bar.

I believe it's changed in OS X Mountain Lion, too. It can be observed by copying a large file in Finder.app.