Share Model merge; Merge initializers; undo/redo descs (#65)

- use "shared" pointer between Model and undo/redo instances
- adds missing initializers to Merge and Text classes
- adds checkpoints for mouse resize, barcode, and image reset.
- renames the position dialog checkpoint from Move -> Position to differentiate it from a mouse move.
- adds the Undo/Redo action descriptions to the menu items.
- adds unit tests for the Merge and Model change, and for ColorNode, RawText and TextNode.

model/unit_tests/TestMerge.h

@@ -0,0 +1,37 @@
+/*  TestMerge.h
+ *
+ *  Copyright (C) 2019  Jim Evins <evins@snaught.com>
+ *
+ *  This file is part of gLabels-qt.
+ *
+ *  gLabels-qt 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 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  gLabels-qt 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 gLabels-qt.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <QtTest/QtTest>
+
+
+class TestMerge : public QObject
+{
+	Q_OBJECT
+
+private slots:
+	void initTestCase();
+	void factory_data();
+	void factory();
+	void factoryNotRegistered();
+	void text_data();
+	void text();
+	void none();
+	void record();
+};