Keybindings: Add a default keybinding for 'switch-panels-backward'.

Auto-shift-reversal for keybindings was eliminated in muffin, so
the forward keybindings that have a default should have their
backward- counterparts set also.

ref: linuxmint/cinnamon#11004

Author: mtwebster

schemas/org.cinnamon.desktop.keybindings.wm.gschema.xml.in

@@ -136,7 +136,7 @@
       <summary>Switch system controls</summary>
     </key>
     <key type="as" name="switch-panels-backward">
-      <default>[]</default>
+      <default><![CDATA[['<Shift><Control><Alt>Tab']]]></default>
       <summary>Reverse switch system controls</summary>
     </key>
     <key type="as" name="show-desktop">