Clipboard support in JS and lots of improvements to editVoxels.js

* Adds "Clipboard" object to JS
* adds Clipboard.copyVoxel(x,y,z,s), Clipboard.cutVoxel(x,y,z,s), Clipboard.deleteVoxel(x,y,z,s), Clipboard.pasteVoxel(x,y,z,s), Clipboard.exportVoxel(x,y,z,s), Clipboard.importVoxels(), Clipboard.nudgeVoxel()
* adds sample JS that demonstrates how to use these features
* editVoxels.js - adds "face edge/line" highlight mode
* editVoxels.js - adds display of tool palette with which tool is active
* editVoxels.js - adds eye dropper/sample support
* editVoxels.js - don't show the voxel preview when in orbit mode
* editVoxels.js - add clicking on color swatches to select the color
* editVoxels.js - adding the voxel size slider and +/- keys to change the editing voxel size

closes #2017 
closes #2032 
closes #2021 
closes #2022 
closes #2019