Add API for system status indicators

Add a container in the panel for holding them, a fast method for
loading and a simple base class to be subclassed by specific
implementations.

https://bugzilla.gnome.org/show_bug.cgi?id=621705

js/ui/panelMenu.js

@@ -35,4 +35,44 @@ Button.prototype = {
         else
             this.actor.remove_style_pseudo_class('pressed');
     }
+};
+
+/* SystemStatusButton:
+ *
+ * This class manages one System Status indicator (network, keyboard,
+ * volume, bluetooth...), which is just a PanelMenuButton with an
+ * icon and a tooltip
+ */
+function SystemStatusButton() {
+    this._init.apply(this, arguments);
+}
+
+SystemStatusButton.prototype = {
+    __proto__: Button.prototype,
+
+    _init: function(iconName,tooltipText) {
+        Button.prototype._init.call(this, St.Align.START);
+        this._iconActor = null;
+        this.setIcon(iconName);
+        this.setTooltip(tooltipText);
+    },
+
+    setIcon: function(iconName) {
+        this._iconName = iconName;
+        if (this._iconActor)
+            this._iconActor.destroy();
+        this._iconActor = St.TextureCache.get_default().load_icon_name(this._iconName, 24);
+        this.actor.set_child(this._iconActor);
+    },
+
+    setTooltip: function(text) {
+        if (text != null) {
+            this.tooltip = text;
+            this.actor.has_tooltip = true;
+            this.actor.tooltip_text = text;
+        } else {
+            this.actor.has_tooltip = false;
+            this.tooltip = null;
+        }
+    }
 };