Add visualisation of the geometry in the XY, YZ and XZ cross-sections

[grzanka]

Right now all the cross-sections are completely empty...

One could draw there at least the border of the volumes

[kmichalikk]

Done some investigation.
The clipped view is instantiated in

ui/src/ThreeEditor/js/viewport/Viewport.js

Line 120 in 5d0d489

viewClipPlane = new ViewportClippedViewCSG(

and it is provided 3 signals: zoneGeometryChanged, zoneAdded, zoneRemoved. Based on the names, this is clearly meant for boolean zones which Geant4 is not using. However, the values dispatched with this signals are plain Object3D objects, not necessarily Zones. These signals are accessible from editor object and can be dispatched anywhere:

this.editor.signals.zoneGeometryChanged.dispatch(geant4Figure);

For this to work, we need to find a sensible place to call these signals: when object is added/removed from scene or changed. I temporarily put the above call in toSerialized method in Geant4Box, Geant4Sphere and Geant4Cylinder, but it shouldn't be there in the final implementation. The code for CSG mostly works, but has some visual glitches that need to be sorted out.

clipping-planes.webm

[grzanka]

Very nice work ! Screencast gives a hope for good implementation