screenshot-window-sizer: Add explicit keybinding for backward cycling

The REVERSES flag was removed from Meta.KeyBindingFlags a while ago, as
gnome-control-center doesn't recognize it and the corresponding "magic"
shift handling. That is, nowadays reversible keybindings need to
provide an explicit reversed binding.

https://bugzilla.gnome.org/show_bug.cgi?id=784079

extensions/screenshot-window-sizer/extension.js

@@ -147,11 +147,18 @@ function init() {
 function enable() {
     Main.wm.addKeybinding('cycle-screenshot-sizes',
                           Convenience.getSettings(),
-                          Meta.KeyBindingFlags.PER_WINDOW | Meta.KeyBindingFlags.REVERSES,
+                          Meta.KeyBindingFlags.PER_WINDOW,
+                          Shell.ActionMode.NORMAL,
+                          cycleScreenshotSizes);
+    Main.wm.addKeybinding('cycle-screenshot-sizes-backward',
+                          Convenience.getSettings(),
+                          Meta.KeyBindingFlags.PER_WINDOW |
+                          Meta.KeyBindingFlags.IS_REVERSED,
                           Shell.ActionMode.NORMAL,
                           cycleScreenshotSizes);
 }
 
 function disable() {
     Main.wm.removeKeybinding('cycle-screenshot-sizes');
+    Main.wm.removeKeybinding('cycle-screenshot-sizes-backward');
 }

extensions/screenshot-window-sizer/org.gnome.shell.extensions.screenshot-window-sizer.gschema.xml

@@ -6,5 +6,9 @@
       <default><![CDATA[['<Alt><Control>s']]]></default>
       <summary>Cycle Screenshot Sizes</summary>
     </key>
+    <key type="as" name="cycle-screenshot-sizes-backward">
+      <default><![CDATA[['<Shift><Alt><Control>s']]]></default>
+      <summary>Cycle Screenshot Sizes Backward</summary>
+    </key>
   </schema>
 </schemalist>