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.

glabels/LabelEditor.cpp

@@ -661,6 +661,7 @@ namespace glabels
 				break;
 
 			case ArrowResize:
+				mUndoRedoModel->checkpoint( tr("Resize") );
 				handleResizeMotion( xWorld, yWorld );
 				break;