* arrow.scm
  http://registry.gimp.org/node/20269
  by B-Ranger (ICQ #52712314, Jabber/XMPP Hoffmann.P {_AT_} jabber.ccc.de)
  See arrow.png for documentation.
  See also http://registry.gimp.org/node/28566 for curved arrows.

  Note that to control the brush thickness ("brush radius", in
  Gimp-speak) you'll have to set it on the toolbox brush or pencil
  tool.  Setting it on the Tools->Arrow control panel won't work,
  although it's obviously supposed to work; it'll just be ignored.
  A good arrowhead length ratio is 3x the line thickness.  I usually
  use lines of 5px and arrowheads of 15px.