Pior to this change when the user added text after having saved the file
the Text Editor wouldn't enable the modified flag, unless this new text
was a new line.
This happened because the UndoStack was merging the Command added by
the new text with the old text one, and when is_current_modified()
was called, the m_stack_index would not have been incremented, and
it would return false.
In this change was added a condition to verify if the modified tag is
active, and the merge is only done if the document is already modified.
964249a5b0