panel: Add function to close the calendar

We will soon replace regular menu items in the calendar drop-down
with more complex elements.
However there will still be items that should close the drop down
when activated - rather than making the menu available throughout
the hierarchy (and eventually from outside as well when we add the
notifications list), have a public method on a global object just
like the ubiquitous Main.overview.hide().

https://bugzilla.gnome.org/show_bug.cgi?id=744817

js/ui/panel.js

@@ -891,6 +891,18 @@ const Panel = new Lang.Class({
             menu.actor.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
     },
 
+    closeCalendar: function() {
+        let indicator = this.statusArea.dateMenu;
+        if (!indicator) // calendar not supported by current session mode
+            return;
+
+        let menu = indicator.menu;
+        if (!indicator.actor.reactive)
+            return;
+
+        menu.close();
+    },
+
     set boxOpacity(value) {
         let isReactive = value > 0;