Add an Activity Chooser to help explain to users what all the activities and resources are

As discussed in http://moodle.org/mod/forum/discuss.php?d=191549, we've written a new module picker for Moodle 2 based on the same design as the Question Picker.

From the positive comments we've received so far, we'd like to submit it for integration into master

Current features of the module chooser (modchooser) include:

• written using YUI3;
• uses progressive enhancement approach;
• displays the icon associated with the module;
• displays any help associated with the module;
• automatically resizes for smaller screens (with a minimum size); and
• supports keyboard access (up/down/tab/space/return of form elements).

The chooser uses the M.core.dialogue (enrol/yui/notification/notification.js) YUI module, and is based upon the Question Bank question type selector in visual design (the Question type selector uses YUI2).