Add support for markers

[fskpf]

E.g. https://developer.mozilla.org/en-US/docs/Web/SVG/Element/marker

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <!-- arrowhead marker definition -->
    <marker id="arrow" viewBox="0 0 10 10" refX="5" refY="5"
        markerWidth="6" markerHeight="6"
        orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 z" />
    </marker>

    <!-- simple dot marker definition -->
    <marker id="dot" viewBox="0 0 10 10" refX="5" refY="5"
        markerWidth="5" markerHeight="5">
      <circle cx="5" cy="5" r="5" fill="red" />
    </marker>
  </defs>

  <!-- Coordinate axes with a arrowhead in both direction -->
  <polyline points="10,10 10,90 90,90" fill="none" stroke="black"
   marker-start="url(#arrow)" marker-end="url(#arrow)"  />

  <!-- Data line with polymarkers -->
  <polyline points="15,80 29,50 43,60 57,30 71,40 85,15" fill="none" stroke="grey"
   marker-start="url(#dot)" marker-mid="url(#dot)"  marker-end="url(#dot)" />
</svg>

[fskpf]

Pushed some basic marker support (wrong issue number of course...).

Still open:

• Implementation for paths
• orientation attribute
• scaling of marker seems to be off

[fskpf]

I've added markers for paths. Guess there are still some problems here and there, but we should create separate issues for then.