MDL-51247 forms: All new aria-pimped autocomplete mform element.

Supports static list of options - or options fetched via ajax.
Has options for single,multi and tags support.

lang/en/form.php

@@ -49,13 +49,15 @@
 $string['mustbeoverriden'] = 'Abstract form_definition() method in class {$a} must be overridden, please fix the code.';
 $string['nomethodforaddinghelpbutton'] = 'There is no method for adding a help button to form element {$a->name} (class {$a->classname})';
 $string['nonexistentformelements'] = 'Trying to add help buttons to non-existent form elements : {$a}';
+$string['noselection'] = 'No selection';
 $string['optional'] = 'Optional';
 $string['othersettings'] = 'Other settings';
 $string['requiredelement'] = 'Required field';
 $string['revealpassword'] = 'Reveal';
 $string['security'] = 'Security';
 $string['selectallornone'] = 'Select all/none';
 $string['selected'] = 'Selected';
+$string['selecteditems'] = 'Selected items:';
 $string['showadvanced'] = 'Show advanced';
 $string['showless'] = 'Show less...';
 $string['showmore'] = 'Show more...';