Constructs a new mailboxes widget with a bundle of images.
tree = new Tree(images);
TreeItem root = new TreeItem(
imageItemHTML(images.home(), "foo@example.com"));
tree.addItem(root);
addImageItem(root, "Inbox", images.inbox());
addImageItem(root, "Drafts", images.drafts());
addImageItem(root, "Templates", images.templates());
addImageItem(root, "Sent", images.sent());
addImageItem(root, "Trash", images.trash());
root.setState(true);
initWidget(tree);