Magnifier: Implement focus and caret tracking

A11y users who use the magnifier may have trouble
focusing when they're typing or trying to keynav.
Implement a new system so that they can have the
magnifier track the caret and focus instead instead
of just the mouse.

Bug https://bugzilla.gnome.org/show_bug.cgi?id=647074

js/ui/focusCaretTracker.js

@@ -0,0 +1,68 @@
+/** -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+/*
+ * Copyright 2012 Inclusive Design Research Centre, OCAD University.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author:
+ *   Joseph Scheuhammer <clown@alum.mit.edu>
+ * Contributor:
+ *   Magdalen Berns <m.berns@sms.ed.ac.uk>
+ */
+
+const Atspi = imports.gi.Atspi;
+const Lang = imports.lang;
+const Signals = imports.signals;
+
+const CARETMOVED        = 'object:text-caret-moved';
+const STATECHANGED      = 'object:state-changed';
+
+const FocusCaretTracker = new Lang.Class({
+    Name: 'FocusCaretTracker',
+
+    _init: function() {
+        Atspi.init();
+        this._atspiListener = Atspi.EventListener.new(Lang.bind(this, this._onChanged));
+    },
+
+    _onChanged: function(event) {
+        let update = null;
+
+        if (event.type.indexOf(STATECHANGED) == 0)
+            update = 'focus-changed';
+        else if (event.type == CARETMOVED)
+            update = 'caret-moved';
+        if(update != null)
+            this.emit(update, event);
+    },
+
+    registerFocusListener: function() {
+        return this._atspiListener.register(STATECHANGED + ':focused') &&
+               this._atspiListener.register(STATECHANGED + ':selected');
+    },
+
+    registerCaretListener: function() {
+        return this._atspiListener.register(CARETMOVED);
+    },
+
+    deregisterFocusListener: function() {
+        return this._atspiListener.deregister(STATECHANGED + ':focused') &&
+               this._atspiListener.deregister(STATECHANGED + ':selected');
+    },
+
+    deregisterCaretListener: function() {
+        return this._atspiListener.deregister(CARETMOVED);
+    }
+});
+Signals.addSignalMethods(FocusCaretTracker.prototype);