Editable regions

One thing that would be extremely useful for embedded editors (see #1248) is defining regions of the source that should be considered editable, and others that should be considered fixed. This could be done in CodeMirror by adding a listener for key presses, which would check if the current selections and insertion points are all in editable regions, and set the read-only bit on the document accordingly. At the same time, the lexer could set a special style on tokens in the non-editable range, and that could be associated with a CSS style that grays out these areas, so the student sees which sections of the code are editable.

Start by examining the CodeMirror integration for embedded editors, then trace how key-press handling, the lexer, and CSS styles are connected. Define how editable regions are specified and how read-only behavior and visual styling should work; the issue names no files or tests, so the implementation path needs project research first.

Scritto dal modello di indicizzazione a partire dal testo della issue.