Don't set modified flag when file on disk is newer

Flagging the document as modified makes it too easy to accidentally overwrite the document - e.g. when closing you're reminded to save it. It also makes it slower to close documents you no longer want open. Flagging as modified is still done if the user cancels the warning.

Also change 'Save' button to 'Overwrite' for clarity.
Here we pass `TRUE` to force saving to disk, because the document may not be modified.