ArrowHead Type Property Change For Next GPML Release

[Finterly]

Because only End Points have actual ArrowHeadTypes, in future GPML: move arrowHead property from Point to Interaction/GraphicalLine.

Now:

<Interaction ">
  <WayPoints>
      <Point x="640.0" y="191.0" arrowHead="Directed"/>
     <Point x="700.0" y="200.0"  arrowHead="Undirected"/>
      <Point x="831.0" y="384.0" arrowHead ="Directed"/>
     ... 

Want:

<Interaction startArrowHead="Directed" endArrowHead="Directed">
  <WayPoints>
    <Point x="640.0" y="191.0" />
    <Point x="700.0" y="200.0"/>
   <Point x="831.0" y="384.0"/>
    ...