Rationalize mouse cursor shape handling

Now can use the full range of standard mouse cursor shapes similar to
those supported by browsers via the CSS cursor property.

Still needs to be fully implemented for cocoa backend.

gen/__main__.py

@@ -32,6 +32,9 @@ def main(args: List[str]=sys.argv) -> None:
     elif which == 'wcwidth':
         from gen.wcwidth import main
         main(args)
+    elif which == 'cursors':
+        from gen.cursors import main
+        main(args)
     else:
         raise SystemExit(f'Unknown which: {which}')