[graph] Manipulation of edge control points

[JanKoehnlein]

sprotty currently supports polyline edges. In the following, we refer to the inner points of the polyline as the control points (CPs). We leave the start and endpoint to a separate issue.

CPs of edges should be editable

• [x] SControlPoint should become an SModel class
• [x] SEdge must be be selectable
• [x] on selection of an edge, its CPs should appear as circular handles
• [x] on deselection of an edge, its CPs should disappear
• [x] CPs should be selectable as well
• [x] CPs should be selectable and moveable
• [x] It should be possible to remove control points from an edge
• [x] It should be possible to add a control point to an edge

Feel free to break this down into smaller Github issues.

[jbicker]

Behaviour

• Simple click on an edge brings it into edit mode, the routing points are showing up
• additionally volatile routing points are showing up between actual existing routing points
• dragging a volatile routing point handle creates an actual routing point (if the angle gets pointed enough)
• double click on a routing point removes it
• selecting routing points and hit backspace key removes them

Further thoughts

• right click on a routing point shows a routing point context menu where one can choose between kinds of routing point (bezier, polygon, rectangular…)
• right click on an edge segment shows a edge context menu where one can choose between kind of edge

[JanKoehnlein]

Sounds good.

[spoenemann]

Most parts have been solved with #202.

It should be possible to remove control points from an edge → #206.