Add support for new startAngle and endAngle properties to Imagine style Dial

This was partially implemented in https://codereview.qt-project.org/c/qt/qtdeclarative/+/467429/18, but it currently breaks existing usages of the Imagine style, as it cuts out a section of the background image. We should instead add an addition image, e.g. "dial-mask" that is a child of the background image, and is used purely to draw the "progress" of the dial. That way, existing applications can continue using the old fixed start and angle assets, and opt-in to the dynamic angles by altering dial-background and adding dial-mask.

We also need to adapt Sketch file for Dial's customisable start and end angle changes.