Add CSV export to context menu of the Database Structure tab

Add an entry for the CSV export to the context menu of the Database
Structure tab of the main windows as well which preselects the currently
selected table.

When opening the CSV export dialog via the standard menu also preselect
the currently selected table when the Database Structure tab is active.

src/MainWindow.ui

@@ -272,7 +272,7 @@
          <rect>
           <x>0</x>
           <y>0</y>
-          <width>763</width>
+          <width>278</width>
           <height>444</height>
          </rect>
         </property>
@@ -1267,6 +1267,17 @@
     <string>Ctrl+F5</string>
    </property>
   </action>
+  <action name="actionExportCsvPopup">
+   <property name="enabled">
+    <bool>false</bool>
+   </property>
+   <property name="text">
+    <string>Export as CSV file</string>
+   </property>
+   <property name="toolTip">
+    <string>Export table as comma separated values file</string>
+   </property>
+  </action>
  </widget>
  <customwidgets>
   <customwidget>
@@ -1983,6 +1994,22 @@
     </hint>
    </hints>
   </connection>
+  <connection>
+   <sender>actionExportCsvPopup</sender>
+   <signal>triggered()</signal>
+   <receiver>MainWindow</receiver>
+   <slot>exportTableToCSV()</slot>
+   <hints>
+    <hint type="sourcelabel">
+     <x>-1</x>
+     <y>-1</y>
+    </hint>
+    <hint type="destinationlabel">
+     <x>399</x>
+     <y>299</y>
+    </hint>
+   </hints>
+  </connection>
  </connections>
  <slots>
   <slot>fileOpen()</slot>