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.

backends/merge/Text.cpp

@@ -45,7 +45,7 @@ namespace glabels
 		Text::Text( const Text* merge )
 			: Merge( merge ),
 			  mDelimeter(merge->mDelimeter), mLine1HasKeys(merge->mLine1HasKeys),
-			  mNFieldsMax(merge->mNFieldsMax)
+			  mKeys(merge->mKeys), mNFieldsMax(merge->mNFieldsMax)
 		{
 		}