data: Add button-layout to override schema

The default in gsettings-desktop-schemas has changed to not include
minimize and maximize to match the default of client-side decorations
in GTK+, so start overriding the setting to bring them back in classic
mode.

data/org.gnome.shell.extensions.classic-overrides.gschema.xml.in

@@ -11,6 +11,14 @@
       </_description>
     </key>
 
+    <key name="button-layout" type="s">
+      <default>"appmenu:minimize,maximize,close"</default>
+      <_summary>Arrangement of buttons on the titlebar</_summary>
+      <_description>
+        This key overrides the key in org.gnome.desktop.wm.preferences when running GNOME Shell.
+      </_description>
+    </key>
+
     <key name="edge-tiling" type="b">
       <default>true</default>
       <_summary>Enable edge tiling when dropping windows on screen edges</_summary>