MDL-30637 Simplify moodle forms

Make the forms easier to use and navigate by adding functionality to
collapse and extend form sections (headers). The logic is as follows:

If form contains 2 and less sections (headers):

* Display the form as non-collapsible at all.
* The point above can be overridden if developer marks the section as expanded
  in form definition (e.g. $mform->setExpanded('foo'));

If form contains 3 and more sections (headers):

* always expanding the first section and closing all others by default;
* always expanding a section containing at least one "required" element;
* expanding any section which contains validation errors after submission;
* expanding any section which was previously open on previous submit (e.g. when
  adding new choices);
* expanding the section which is marked as expanded in form definition (e.g.
  $mform->setExpanded('foo');

lib/form/yui/shortforms/shortforms.js

@@ -0,0 +1,96 @@
+YUI.add('moodle-form-shortforms', function(Y) {
+    /**
+     * Provides the form shortforms class
+     *
+     * @module moodle-form-shortforms
+     */
+
+    /**
+     * A class for a shortforms
+     *
+     * @param {Object} config Object literal specifying shortforms configuration properties.
+     * @class M.form.shortforms
+     * @constructor
+     * @extends Y.Base
+     */
+    function SHORTFORMS(config) {
+        SHORTFORMS.superclass.constructor.apply(this, [config]);
+    }
+
+    var SELECTORS = {
+            FIELDSETCOLLAPSIBLE : 'fieldset.collapsible',
+            LEGENDFTOGGLER : 'legend.ftoggler'
+        },
+        CSS = {
+            COLLAPSED : 'collapsed',
+            FHEADER : 'fheader',
+            JSPROCESSED : 'jsprocessed'
+        },
+        ATTRS = {};
+
+    /**
+     * Static property provides a string to identify the JavaScript class.
+     *
+     * @property NAME
+     * @type String
+     * @static
+     */
+    SHORTFORMS.NAME = 'moodle-form-shortforms';
+
+    /**
+     * Static property used to define the default attribute configuration for the Shortform.
+     *
+     * @property ATTRS
+     * @type String
+     * @static
+     */
+    SHORTFORMS.ATTRS = ATTRS;
+
+    /**
+     * The form ID attribute definition
+     *
+     * @attribute formid
+     * @type String
+     * @default ''
+     * @writeOnce
+     */
+    ATTRS.formid = {
+        value : null
+    };
+
+    Y.extend(SHORTFORMS, Y.Base, {
+        initializer : function() {
+            var fieldlist = Y.Node.all('#'+this.get('formid')+' '+SELECTORS.FIELDSETCOLLAPSIBLE);
+            // Look through collapsible fieldset divs
+            fieldlist.each(this.process_fieldset, this);
+            // Subscribe collapsible fieldsets to click event
+            Y.one('#'+this.get('formid')).delegate('click', this.switch_state, SELECTORS.FIELDSETCOLLAPSIBLE+' .'+CSS.FHEADER);
+        },
+        process_fieldset : function(fieldset) {
+            fieldset.addClass(CSS.JSPROCESSED);
+            // Get legend element
+            var legendelement = fieldset.one(SELECTORS.LEGENDFTOGGLER);
+
+            // Turn headers to links for accessibility
+            var headerlink = Y.Node.create('<a href="#"></a>');
+            headerlink.addClass(CSS.FHEADER);
+            headerlink.appendChild(legendelement.get('firstChild'));
+            legendelement.prepend(headerlink);
+        },
+        switch_state : function(e) {
+            e.preventDefault();
+            var fieldset = this.ancestor(SELECTORS.FIELDSETCOLLAPSIBLE);
+            // toggle collapsed class
+            fieldset.toggleClass(CSS.COLLAPSED);
+            // get corresponding hidden variable
+            var statuselement = new Y.one('input[name=mform_isexpanded_'+fieldset.get('id')+']');
+            // and invert it
+            statuselement.set('value', Math.abs(Number(statuselement.get('value'))-1));
+        }
+    });
+
+    M.form = M.form || {};
+    M.form.shortforms = M.form.shortforms || function(params) {
+        return new SHORTFORMS(params);
+    };
+}, '@VERSION@', {requires:['base', 'node', 'selector-css3']});