Resolve cyclic dependency on StatusMenu

Solved by splitting the base class (PanelMenuButton) in a separate
module, ui.panelMenu, which is meant to hold also other reusable
button classes.

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

js/ui/panelMenu.js

@@ -0,0 +1,38 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+const St = imports.gi.St;
+const Lang = imports.lang;
+const PopupMenu = imports.ui.popupMenu;
+const Main = imports.ui.main;
+
+function Button(menuAlignment) {
+    this._init(menuAlignment);
+}
+
+Button.prototype = {
+    _init: function(menuAlignment) {
+        this.actor = new St.Bin({ style_class: 'panel-button',
+                                  reactive: true,
+                                  x_fill: true,
+                                  y_fill: false,
+                                  track_hover: true });
+        this.actor._delegate = this;
+        this.actor.connect('button-press-event', Lang.bind(this, this._onButtonPress));
+        this.menu = new PopupMenu.PopupMenu(this.actor, menuAlignment, St.Side.TOP, /* FIXME */ 0);
+        this.menu.connect('open-state-changed', Lang.bind(this, this._onOpenStateChanged));
+        Main.chrome.addActor(this.menu.actor, { visibleInOverview: true,
+                                                affectsStruts: false });
+        this.menu.actor.hide();
+    },
+
+    _onButtonPress: function(actor, event) {
+        this.menu.toggle();
+    },
+
+    _onOpenStateChanged: function(menu, open) {
+        if (open)
+            this.actor.add_style_pseudo_class('pressed');
+        else
+            this.actor.remove_style_pseudo_class('pressed');
+    }
+};