Introduce SwitchButton

While porting Bluedevil and plasma-nm to PC3, I noticed that both of them were using this control, but with slightly different code, behavior, sizing, etc. Seems like it was copy-pasted from one into the other at some point. So I thought I'd take the opporutnity to extract it into a library component here in plasma-framework.

This also lays the groundwork for replacing the checkbox with a switch at some point in the future once we have a nice-looking switch control.

There are no substantive visual changes compared to the custom versions in the plasna-nm and bluedevil applets