ProgressButton is a custom progress indicator with a tiny footprint.
Based off the sample provided by Roman Nurik. The default implementation provides a pin progress button as seen on the Android design site, and as seen on the official Google Play suite of apps. The color indicates whether it's downloaded (blue) or not (gray). The appearance of the pin indicates whether the download is permanent (white, upright) or temporary (gray, diagonal). When state is in the process of changing, progress is indicated by a moving pie chart.
