Add screen reader keybinding for improve accessibility

The screen reader keybinding was not set as default and was one of the
things making difficult have good accessibility support in cinnamon.
This commit will add keybinding Alt+Super+s like the gnome one,
improving slightly the accessibility.

Author: Fantu

schemas/org.cinnamon.desktop.keybindings.media-keys.gschema.xml.in.in

@@ -142,7 +142,7 @@
       <_description>Binding to launch the web browser.</_description>
     </key>
     <key name="screenreader" type="as">
-      <default>['']</default>
+      <default>['&lt;Alt&gt;&lt;Super&gt;s']</default>
       <_summary>Toggle screen reader</_summary>
       <_description>Binding to start the screen reader</_description>
     </key>