Example tool to clip view based on element geometry

[pmconne]

Fixes #3883.
Select one or more elements. The backend produces polyfaces from their geometry. We create a ClipVector from those polyfaces and apply it to the view.

TODO:

• Add support for applying an offset to the polyfaces to expand them outward @EarlinLutz has WIP API on EDL/MeshOffset branch - not yet available.
• Integrate V-HACD.js to perform convex mesh decomposition.
  • Ideally, do this in a WebWorker for demo purposes probably won't bother.
• Add a keyin to configure the tolerances, mesh offset, and decomposition parameters. meh.
• An API to ask whether a polyface is already convex, to avoid potentially expensive decomposition? none such exists.

[bbastings]

New tool looks good, could probably have sub-classed from ViewClipByElementTool and just overridden doClipToElements and toolId. :)