Editor: disable "Save" button when code is not modified

[Psychophanta]

I think this is a minor change in the editor functionallity, but interesting one:
In the PB editor, when a source code is modified and not saved to disk, there appears in the panel gadget, the file name with an asterisk at the end, but the button icon of "save" is not disabled.
May be interesting the functionallity of disable this button when source code is not modified, and enable it only if code is modified and not saved (exactly when asterisk does appear) just as happens with "openoffice", "libreoffice", etc.