Bring back separate Save and Save As

This will allow the user to decide whether or not to open the file
chooser. Surprisingly, the code for both of these still exists -- it was
already possible to choose between them using keyboard shortcuts -- so
all we need to do is edit the menu.

data/gnome-chess.ui

@@ -9,6 +9,10 @@
       </item>
       <item>
         <attribute name="label" translatable="yes">_Save Game</attribute>
+        <attribute name="action">win.save</attribute>
+      </item>
+      <item>
+        <attribute name="label" translatable="yes">_Save Game As…</attribute>
         <attribute name="action">win.save-as</attribute>
       </item>
     </section>