Add entity grid tool

This adds a `WebWindowClass` object that allows a script to create a window with a WebView inside, and communicate to it via asynchronous calls (slots+signals).  Feedback appreciated.
I was going to wait to put this back in, but I'll go ahead and just uncomment it to bring it back in.  I just tested it and it works well enough, although it could use some refinement when grid snapping is active.