Add selectable message box helper, use it, make login GUI error label selectable

gui/CustomMessageBox.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <QMessageBox>
+
+namespace CustomMessageBox
+{
+	QMessageBox *selectable(QWidget *parent, const QString &title, const QString &text,
+							QMessageBox::Icon icon = QMessageBox::NoIcon,
+							QMessageBox::StandardButtons buttons = QMessageBox::Ok,
+							QMessageBox::StandardButton defaultButton = QMessageBox::NoButton);
+}