Arrows for directed links

[remikey]

Please implement an extra LinkPainter which features arrow tips for directed links.

[remikey]

What's been done:

• Created jbotsim-extras-common/io.jbotsim.ui.painting.ArrowTipPathGenerator

  Creates a path corresponding to an arrow tip.

• Created jbotsim-extras-swing/io.jbotsim.ui.painting.JDirectedLinkPainter

  Uses a ArrowTipPathGenerator to generate an arrow-tip for directed links and fills it.
  It is currently in jbotsim-extras-swing since it might consume more resources than the behavior already present in the JLinkPainter.

• Added an example in examples.basic.links.DirectedLinksMain

[remikey]

merged into develop.