Second steps with user interface

I have refined the test program in the previous post to add points. So now you can both add and delete as well as move points. Some would argue that this is enough. But there are some tools that would greatly speedup editing that no one else seems to have thought about yet:

1. Slicing a polygon in two. Imagine that you have a polygon that covers several words. You need to cut it quickly into two. With just the ability to delete and add points to existing polygons (no new polygons) how else can you do that? With the mouse all you would need to do would be to drag a line over an anchored polygon, then release the mouse to slice it along that line.
   
2. Merge two or more polygons. If you have fragmented polygons it would be great to just shift-click them and then merged them in a single stroke of the mouse. This could be done by dragging from inside one of the polygons and dragging across the ones to be merged, ending inside another polygon. Then all dragged-over polygons could be merged.
   
3. Create blobs. By clicking on a region that has no polygon you could send a message to the service to try to recognise a word in one go.
   

I've nearly got 1. to work.1 and 2 are a bit counter-intuitive because dragging in drawing programs is supposed to draw a square marquee. But marquees are just not very useful in this case, so I think overriding the default is a good idea. We need to facilitate the operations that the editor of a set of polygons will use all the time, or it will quickly become tedious.