Fix issue where the `this` reference in the double-click handler
callback was incorrectly bound to the graph instance instead of the
editor instance, breaking compatibility with mxGraph behavior.

Changes:
- Store editor reference explicitly using `const editor = this`
- Use `editor` instead of `this` in the callback to make intent clear
- Add test to verify correct editor context in double-click handler

This matches the mxGraph pattern where `mxUtils.bind(this, ...)` was
used to ensure the callback had the correct editor context.