Syntax editor: in-place replacement of functions

When at a non-leaf node, refinements with identical type signatures
are highlighting and can re placed without destroying the children.
If not, the refinement is greyed and the user is asked to clear
the current subtree first if they wish to replace it.
This aspect of the UI should be polished, but at least it is obvious.
Also, some substantial optimizations can still be made to cache
the processed type signatures (which determine what can be replaced
in-place)

src/www/syntax-editor/README.md

@@ -29,12 +29,15 @@ An improved version of the [old syntax editor][1].
 
 See `editor.html` and `editor_online.js`.
 
+## Bugs
+
+- Change startcat doesn't work when given an initial startcat 
+
 ## TODO
 
 - Wrap a subtree
 - Compatibility with grammars with dependent category types
 - Clicking on tokens to select tree node
-- try to retain subtree when replacing node
 - Use local caching
 - Clipboard of trees
 - Usage of printnames