update enabled status of undo/redo actions in the editor * file-editor-tab.h: new inline function returning the qscintilla edit area * octave-qscintilla.cc (constructor): connect textChanged signal to new private slot text_changed; (text_changed): new slot emitting a signal on undo/redo availability * octave-qscintilla.h: new slot and new signal * file-editor.cc (edit_status_update): new slot for updating undo/redo actions; (construct): undo/redo actions disabled at the beginning; (add_new_edit_tab): connect new slot to the signal from the qscintilla edit area indicating undo/redo availability * file-editor.h: new slot

--- a/libgui/src/m-editor/file-editor-tab.h
+++ b/libgui/src/m-editor/file-editor-tab.h
@@ -47,6 +47,8 @@
 
   ~file_editor_tab (void);
 
+  octave_qscintilla *qsci_edit_area () { return _edit_area; }
+
 public slots:
 
   void update_window_title (bool modified);