Add pause style to QProgressBar

QProgressBar seems to use the built-in Windows themes for coloring its QProgressBars, so re-coloring the bar itself is quite complex. However, the Aero-style Windows themes support multiple progressbar states, including Normal, Paused, Stopped, and Indeterminate (https://stackoverflow.com/questions/49867207/how-do-i-change-the-state-of-a-qprogressbar).

It'd be really useful if this was exposed in the Qt API via `pause()`, `stop()`, and `resume()` functions, plus associated signals. This already seems to be implemented in QWinTaskbarProgress, so I think most of the code is already there... just needs to be added to QProgressBar. I'd do it myself, but I got lost in the many layers of abstraction in QProgressBar's styling.

I cross-posted this as a question on StackOverflow: https://stackoverflow.com/questions/49867207/how-do-i-change-the-state-of-a-qprogressbar