Unlock key signature when needed, added keyReadOnly property to score object,...

Unlock key signature when needed, added keyReadOnly property to score object, so executor can set it when necessary. Also AccidConrol.qml depends on notify-able cursorAlter property of score object (previously there was opposite) - this way force accidentals by executor could be implemented. Display confirm tip as well. Display fake key sign. - use keyName of MainScore.qml position for it