Add arrow_size and adjust default colors.

* Add option to configure the arrow head size when using start_mark
  or end_mark equal to "arrow".
* Update default color definitions to be less stark. Used the W3
  schools red, yellow, blue color wheel for the color definitions
  of the six primary colors. In addition remove background color
  (this should always be white) and nearwhite colors, these are
  hold overs from WWPlot.
* Disable dragging elements in the JSXGraph board by default
  (labels were draggable).
* Set useMathJax to true for JSXGraph labels.

Author: somiaj

lib/Plots/JSXGraph.pm

@@ -48,9 +48,6 @@ sub HTML {
 				$self->{JS}
 				$self->{JSend}
 				board.unsuspendUpdate();
-				// This is a workaround for an issue when the magnified graph is drawn in the imageview dialog.
-				// In that case something is messing up the bounding box, so it needs to be reset.
-				board.setBoundingBox(board.attr.boundingbox, board.keepaspectratio, 'keep');
 				return board;
 			}