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/ui/MergeView.ui

@@ -95,13 +95,6 @@
         <string>Records</string>
        </property>
        <layout class="QGridLayout" name="gridLayout_4">
-        <item row="0" column="0">
-         <widget class="QTableWidget" name="recordsTable">
-          <property name="focusPolicy">
-           <enum>Qt::NoFocus</enum>
-          </property>
-         </widget>
-        </item>
         <item row="1" column="0">
          <layout class="QHBoxLayout" name="horizontalLayout">
           <item>
@@ -140,6 +133,28 @@
           </item>
          </layout>
         </item>
+        <item row="0" column="0">
+         <widget class="QTableView" name="recordsTableView">
+          <property name="focusPolicy">
+           <enum>Qt::NoFocus</enum>
+          </property>
+          <property name="selectionMode">
+           <enum>QAbstractItemView::NoSelection</enum>
+          </property>
+          <property name="textElideMode">
+           <enum>Qt::ElideRight</enum>
+          </property>
+          <attribute name="horizontalHeaderCascadingSectionResizes">
+           <bool>true</bool>
+          </attribute>
+          <attribute name="horizontalHeaderStretchLastSection">
+           <bool>true</bool>
+          </attribute>
+          <attribute name="verticalHeaderStretchLastSection">
+           <bool>false</bool>
+          </attribute>
+         </widget>
+        </item>
        </layout>
       </widget>
      </item>