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

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() {