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