Selecting dragging machine widgets seems to work.
Added a separate Rubberband class into geometry.h. + Some cleanup in machine_scenes.{h,cpp}.
Added a simple knob.
Fixed some bugs with machine scene and widget.

- dragEnd() was delivered twice to the front scene instead of
  begin delivered to the front and back scenes,
  upon MachineWidget::dragged message.

- Removed the red rectangle artifact when dragging machines
  around the scene. Occurred under certain conditions.
Showing a little knob on a m3 machine. Just for debugging purposes.
Flipped and resized the knob to 0.75 inch.
Using absolute mouse positions for controlling Knobs.
Added some docs.