Add a Switch

Add a Switch element, this shows several problems and features still missing:

• binding on the position of a Union.Background sometimes get broken, even if nothing touches it from qml (not all switches will move the handle on toggle)
• sizing of the widget should come from the size of an hint item called hint-bar-size instead of the natural size of the frame
• quite some logic of the handle moving stays on QML side (probably ok)
• states change size, as adding handle-focus might change the final image size, causing potential alignment problems