Add a toggle-overview keybinding

This is a new, regular keybinding for going to the overview.
The default binding is Super-S, which goes well with Super-A
for going to the application grid.
This is separate from the existing panel-main-menu keybinding,
so that we can keep Alt-F1 opening the main menu in classic mode.

https://bugzilla.gnome.org/show_bug.cgi?id=698251

data/50-gnome-shell-system.xml.in

@@ -11,6 +11,9 @@
 	<KeyListEntry name="focus-active-notification"
                       _description="Focus the active notification"/>
 
+	<KeyListEntry name="toggle-overview"
+                      _description="Show the overview"/>
+
 	<KeyListEntry name="toggle-application-view"
                       _description="Show all applications"/>
 

data/org.gnome.shell.gschema.xml.in.in

@@ -117,6 +117,13 @@ value here is from the GsmPresenceStatus enumeration.</_summary>
         Overview.
       </_description>
     </key>
+    <key name="toggle-overview" type="as">
+      <default>["&lt;Super&gt;s"]</default>
+      <_summary>Keybinding to open the overview</_summary>
+      <_description>
+        Keybinding to open the Activities Overview.
+      </_description>
+    </key>
     <key name="toggle-message-tray" type="as">
       <default>["&lt;Super&gt;m"]</default>
       <_summary>Keybinding to toggle the visibility of the message tray</_summary>

js/ui/viewSelector.js

@@ -150,6 +150,14 @@ const ViewSelector = new Lang.Class({
                               Shell.KeyBindingMode.NORMAL |
                               Shell.KeyBindingMode.OVERVIEW,
                               Lang.bind(this, this._toggleAppsPage));
+
+        Main.wm.addKeybinding('toggle-overview',
+                              new Gio.Settings({ schema: SHELL_KEYBINDINGS_SCHEMA }),
+                              Meta.KeyBindingFlags.NONE,
+                              Shell.KeyBindingMode.NORMAL |
+                              Shell.KeyBindingMode.OVERVIEW,
+                              Lang.bind(Main.overview, Main.overview.toggle));
+
     },
 
     _toggleAppsPage: function() {