Dealing with general click and drag on the applet and differentiating
between a background drag and dragging an element seemed like too big
a task, so this PR simply provides a slider to rotate the diagram when
the pivot is defined.
Implementing this required storing much more construction data, and
also dealing head-on with GeoGebra's shall we say "strange" choice where
the value of an expression depends on what name it is assigned to...
The resolution of this last bit was to use different GeoGebra names for
Geometry Applet points that start with something other than an uppercase
Roman letter.
These include circleSlider, line cutoff, and bichord. Also
makes the order of points in the chord command more true to
Joyce. Represents more progress on #36. Next up is dealing with
the pivot parameter.
