Update checking feature (#2648)

* Check on startup (toggleable setting) and manually
* Option to check for pre-releases (eg, 2.4.0-beta1)
* Only included if WITH_XC_NETWORKING is enabled

src/gui/MainWindow.ui

@@ -223,6 +223,7 @@
      <string>&amp;Help</string>
     </property>
     <addaction name="actionAbout"/>
+    <addaction name="actionCheckForUpdates"/>
     <addaction name="actionDonate"/>
     <addaction name="actionBugReport"/>
    </widget>
@@ -348,6 +349,14 @@
     <enum>QAction::AboutRole</enum>
    </property>
   </action>
+  <action name="actionCheckForUpdates">
+   <property name="text">
+    <string>Check for Updates...</string>
+   </property>
+   <property name="menuRole">
+    <enum>QAction::ApplicationSpecificRole</enum>
+   </property>
+  </action>
   <action name="actionDatabaseOpen">
    <property name="text">
     <string>&amp;Open database...</string>