In MergeView, replace QTableWidget with QTableView with custom model (#266,#217)

QTableWidget was a major bottleneck for large merge sources (#217).  This is because a QTableWidgetItem needed to be created for every field in every record of the merge data, whether they are being displayed or not.  This was not a problem for small merge sources (only a few dozen records max), however for larger data sets this would severely affect performance and make the application unresponsive.  QTableView only renders the fields and records currently visible.

glabels/CMakeLists.txt

@@ -21,6 +21,7 @@ set (glabels_sources
   Help.cpp
   LabelEditor.cpp
   MainWindow.cpp
+  MergeTableModel.cpp
   MergeView.cpp
   MiniPreviewPixmap.cpp
   NotebookUtil.cpp
@@ -57,6 +58,7 @@ set (glabels_qobject_headers
   File.h
   LabelEditor.h
   MainWindow.h
+  MergeTableModel.h
   MergeView.h
   ObjectEditor.h
   PreferencesDialog.h