Edge :
======

   Edges are directed and composed of line segments. They join any
couple of objects with types in {port, web, vertex}. They are multiple
named.

   To create an edge you select the <ObjectsEdit>:<Edge>:<Create (e)>
menu command ('e' is the keyboard shortcut), you click inside the
source object, as long as you maintain the button down, a segment
shadow follow the mouse cursor. If you release the button in a port, a
vertex or a web then the edge is created straight, else a segment is
created and you must continue until you give the target object.

   Points may be added, moved or killed by menu.

   The shape of the edge arrow is fixed by resources: length of the
arrow and angle between the arrow and the edgesegment.

   Like every graphic objects a resource value fixe the clicking zone
arround the edge.

   The menu commands <ObjectsEdit>:<Edge>:<change Source> and
<ObjectsEdit>:<Edge>:<change Target> allow to redirect a given edge.
It is not yet implemented.

   The menu commands <ObjectsEdit>:<Edge>:<Show Source> and
<ObjectsEdit>:<Edge>:<Show Target> highlight the corresponding
objects.

Edge line width can be fixed by resources :
    atg*edgeLineWidth: 0 (mean 1 but draw by fast raster)
