/**
  * Processes the key event pressed.
  *
  * @param keyEvent An event which indicates that a keystroke occurred in a component.
  */
 private void processKeyEventPressed(KeyEvent keyEvent) {
   if (keyEvent.getCode() == KeyCode.CONTROL) {
     controlPressed = true;
   } else if (keyEvent.getCode() == KeyCode.Z) {
     zPressed = true;
   } else if (keyEvent.getCode() == KeyCode.Y) {
     yPressed = true;
   } else if (keyEvent.getCode() == KeyCode.Q) {
     qPressed = true;
   } else if (keyEvent.getCode() == KeyCode.F12) {
     mainApp.showHelpOverview();
   } else if (keyEvent.getCode() == KeyCode.F5) {
     mainApp.showOverdueOverview();
   } else if (keyEvent.getCode() == KeyCode.F4) {
     mainApp.showCompleteOverview();
   } else if (keyEvent.getCode() == KeyCode.F3) {
     mainApp.showUpcomingOverview();
   } else if (keyEvent.getCode() == KeyCode.F2) {
     mainApp.showTodayOverview();
   } else if (keyEvent.getCode() == KeyCode.F1) {
     mainApp.showTaskOverview();
   } else if (keyEvent.getCode() == KeyCode.F11) {
     mainApp.getPrimaryStage().toBack();
   } else if (keyEvent.getCode() == KeyCode.ESCAPE) {
     commandText.setText("home");
     onEnter();
   }
   if (controlPressed && zPressed) {
     commandText.setText("undo");
     onEnter();
   }
   if (controlPressed && yPressed) {
     commandText.setText("redo");
     onEnter();
   }
   if (controlPressed && qPressed) {
     System.exit(0);
   }
 }
 /** When all tasks tab is clicked, show task overview. */
 @FXML
 void onClickedAllTask() {
   mainApp.showTaskOverview();
 }