ProgressBar: implement styling for indeterminate mode

The existing Breeze QStyle implementation uses a series of moving
rectangles that are clipped into a rectangle with rounded corners.
Unfortunately we can't reimplement this because QML does not support
non-rectangular clipping.
See https://bugreports.qt.io/browse/QTBUG-9008.

Instead we use a series of moving pills, which looks quite nice IMO.