Add the vectorscope plugin (#5328)

The credit for the `ColorChooser` class goes to CYBERDEViLNL.

include/ColorChooser.h

@@ -0,0 +1,41 @@
+/* ColorChooser.h - declaration and definition of ColorChooser class.
+ *
+ * Copyright (c) 2019 CYBERDEViLNL <cyberdevilnl/at/protonmail/dot/ch>
+ *
+ * This file is part of LMMS - https://lmms.io
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program (see COPYING); if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301 USA.
+ *
+ */
+
+#include <QColorDialog>
+#include <QApplication>
+#include <QKeyEvent>
+
+class ColorChooser: public QColorDialog
+{
+public:
+	ColorChooser(const QColor &initial, QWidget *parent): QColorDialog(initial, parent) {};
+	ColorChooser(QWidget *parent): QColorDialog(parent) {};
+
+protected:
+	// Forward key events to the parent to prevent stuck notes when the dialog gets focus
+	void keyReleaseEvent(QKeyEvent *event) override
+	{
+		QKeyEvent ke(*event);
+		QApplication::sendEvent(parentWidget(), &ke);
+	}
+};