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/Merge.cpp

@@ -31,7 +31,7 @@ namespace glabels
 		///
 		/// Constructor
 		///
-		Merge::Merge( const Merge* merge ) : mSource(merge->mSource)
+		Merge::Merge( const Merge* merge ) : mId(merge->mId), mSource(merge->mSource)
 		{
 			foreach ( Record* record, merge->mRecordList )
 			{