Support additional Shapes

[dfreeman06]

• e.g. circles / triangles
• implications of edge connections
• different arrowheads

[nrbgt]

Regarding ◀️ : doable, but require <defs><marker id="foo">, and can then be referenced in the style="marker-end:url(#foo)" of a path (or this can be set by css rules and classes, e.g. elk-line-marker-arrow-open).

Looks like we can overload SGraphView and inject the defs.

This would complicate offline SVG generation, which just blindly strips ids (as they weren't useful yet) but we can mark them with classes, e.g. class="elk-preserve-id".... or whitelist/mangle anything that appears in use(#*)