Scriptable Overlay Support

* adds 3D cube overlays, useful for voxel visualization tools like add/delete/select voxel
* adds 3D sphere overlays, might be useful for editing tools or collision proxy visualizations
* adds 3D line overlays, useful for voxel editing tools
* adds 2D image overlays, useful for tool palettes, etc
* adds support for finding the overlay under a point (to allow you to click on an overlay)
* adds 2D text overlay support
* example script shows how to implement color swatches and sliderbar and text feedback

closes #2004, closes #2005, closes #1912, closes #1913