Add setOpacity to guimessagedialog

src/guimessagedialog.cpp

@@ -101,3 +101,7 @@ void GUIMessageDialog::hideExitButton() {
     ui->btn_main->setVisible(false);
     disconnect(ui->btn_main,0,0,0);
 }
+
+void GUIMessageDialog::setOpacity(double opacity) {
+    this->setWindowOpacity(opacity);
+}