Port knob

This ports the footils/knob object to display properly in the Purr Data GUI.

For compatibility we port it as flatgui/knob, which is not loaded at startup by default. I don't think footils/knob was ever shipped with Pd-extended while the former was.

This is bug compatible with footils/knob. For example, the background color is not used to fill in the area of the circle. (knob.c had a tk canvas command attempting to do this, but it used two conflicting tk flags that ended up not painting the circle per tk's documentation.)