Remove the Code Editor example

The example only shows how to put one widget into another widget, and how to use QTextBlock to paint the contents of a QPlainTextEdit in a subclass.

However, the architecture of the example is quite horrible; using a separate widget to paint the line numbers overcomplicates the code, to the point where it becomes a hack to circumvent that some functionality is protected.

The only useful part of the example is the painting of the text document, which could be a snippet in e.g. QTextBlock.