Allow opening databases in read only mode

See issue #325.

src/MainWindow.ui

@@ -873,6 +873,7 @@
     </widget>
     <addaction name="fileNewAction"/>
     <addaction name="fileOpenAction"/>
+    <addaction name="fileOpenReadOnlyAction"/>
     <addaction name="fileAttachAction"/>
     <addaction name="fileCloseAction"/>
     <addaction name="separator"/>
@@ -1993,6 +1994,27 @@
     <string>F5</string>
    </property>
   </action>
+  <action name="fileOpenReadOnlyAction">
+   <property name="icon">
+    <iconset resource="icons/icons.qrc">
+     <normaloff>:/icons/db_open</normaloff>:/icons/db_open</iconset>
+   </property>
+   <property name="text">
+    <string>&amp;Open Database Read Only...</string>
+   </property>
+   <property name="toolTip">
+    <string>Open an existing database file in read only mode</string>
+   </property>
+   <property name="statusTip">
+    <string>Open an existing database file</string>
+   </property>
+   <property name="whatsThis">
+    <string>This option is used to open an existing database file.</string>
+   </property>
+   <property name="menuRole">
+    <enum>QAction::NoRole</enum>
+   </property>
+  </action>
  </widget>
  <customwidgets>
   <customwidget>
@@ -3039,6 +3061,22 @@
     </hint>
    </hints>
   </connection>
+  <connection>
+   <sender>fileOpenReadOnlyAction</sender>
+   <signal>triggered()</signal>
+   <receiver>MainWindow</receiver>
+   <slot>fileOpenReadOnly()</slot>
+   <hints>
+    <hint type="sourcelabel">
+     <x>-1</x>
+     <y>-1</y>
+    </hint>
+    <hint type="destinationlabel">
+     <x>518</x>
+     <y>314</y>
+    </hint>
+   </hints>
+  </connection>
  </connections>
  <slots>
   <slot>fileOpen()</slot>
@@ -3101,5 +3139,6 @@
   <slot>browseDataSetDefaultTableEncoding()</slot>
   <slot>browseDataFetchAllData()</slot>
   <slot>exportTableToJson()</slot>
+  <slot>fileOpenReadOnly()</slot>
  </slots>
 </ui>