First cut at activities overlay

When the user clicks on "Activities", adjust the input shape
to the whole screen and show a black overlay group. Actually, the
black should be *beneath* the window actors and the overlay group
transparent so we can fade in the black while leaving the windows
unfaded, visible, but shrunk and rearranged.

svn path=/trunk/; revision=14

js/ui/overlay.js

@@ -0,0 +1,47 @@
+/* -*- mode: js2; js2-basic-offset: 4; -*- */
+
+const Shell = imports.gi.Shell;
+const Clutter = imports.gi.Clutter;
+
+const Panel = imports.ui.panel;
+
+const OVERLAY_BACKGROUND_COLOR = new Clutter.Color();
+OVERLAY_BACKGROUND_COLOR.from_pixel(0x000000ff);
+
+function Overlay() {
+    this._init();
+}
+
+Overlay.prototype = {
+    _init : function() {
+	let global = Shell.global_get();
+
+	this._group = new Clutter.Group();
+	this.visible = false;
+
+	let background = new Clutter.Rectangle({ color: OVERLAY_BACKGROUND_COLOR,
+						 reactive: true,
+						 x: 0,
+						 y: Panel.PANEL_HEIGHT,
+					         width: global.screen_width,
+					         height: global.screen_width - Panel.PANEL_HEIGHT });
+	this._group.add_actor(background);
+
+	this._group.hide();
+	global.overlay_group.add_actor(this._group);
+    },
+
+    show : function() {
+	if (!this.visible) {
+	    this.visible = true;
+	    this._group.show();
+	}
+    },
+
+    hide : function() {
+	if (this.visible) {
+	    this.visible = false;
+	    this._group.hide();
+	}
+    }
+};