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

gui/CustomMessageBox.cpp

@@ -0,0 +1,19 @@
+#include "CustomMessageBox.h"
+
+namespace CustomMessageBox
+{
+	QMessageBox *selectable(QWidget *parent, const QString &title, const QString &text,
+							QMessageBox::Icon icon,	QMessageBox::StandardButtons buttons,
+							QMessageBox::StandardButton defaultButton)
+	{
+		QMessageBox *messageBox = new QMessageBox(parent);
+		messageBox->setWindowTitle(title);
+		messageBox->setText(text);
+		messageBox->setStandardButtons(buttons);
+		messageBox->setDefaultButton(defaultButton);
+		messageBox->setTextInteractionFlags(Qt::TextSelectableByMouse);
+		messageBox->setIcon(icon);
+
+		return messageBox;
+	}
+}