MDL-54633 tinymce: Automatically sync editor with its textarea

Probably as a result of recent changes in the way how forms client side
validators are trigerred (MDL-52826), the field validator has been
triggered before the underlying textarea's values property is updated by
TinyMCE. This led to marking such a field as "required" even if the
value is provided.

Inspired by http://stackoverflow.com/questions/2122085/
this patch adds a new onchange callback that automatically keeps the
underlying textarea synced with the editor iframe. Relevant API docs:
http://archive.tinymce.com/wiki.php/Configuration3x:onchange_callback

I was also trying to call the save() method via the editor's onSubmit
method but that one seems to be also triggered only after the validator.

lib/editor/tinymce/module.js

@@ -45,6 +45,7 @@ M.editor_tinymce.init_editor = function(Y, editorid, options) {
         M.editor_tinymce.initialised = true;
         M.util.js_pending('editors');
         options.oninit = "M.editor_tinymce.init_callback";
+        options.onchange_callback = "M.editor_tinymce.onchange_callback";
     }
 
     M.editor_tinymce.editor_options[editorid] = options;
@@ -92,6 +93,12 @@ M.editor_tinymce.init_callback = function() {
     M.util.js_complete('editors');
 }
 
+M.editor_tinymce.onchange_callback = function(editorinstance) {
+    // We need to keep the underlying textarea in sync so that when the form
+    // validator is triggered, it has the value property up-to-date.
+    editorinstance.save();
+};
+
 M.editor_tinymce.init_filepicker = function(Y, editorid, options) {
     M.editor_tinymce.filepicker_options[editorid] = options;
 };