[FIX] web_editor: reset snippet editors on snippet move

As indicated in a code comment, destroying and rebuilding all snippet
editors when a snippet is moved is more accurate as the snippet options
may depend on the new locations in a way the 'onMove' callback cannot
handle. That operation seemed however too costly for no visual benefit
at the time. Now, not implementing it is a source of bugs and redundant
code (as with the new "number of columns" option which is not properly
updated once a column is moved in another row), so this commit
implements the feature.

This also allows for some improvement: the 'cleanForSave' part of
snippet editors is always performed when they are destroyed.

If this proves to be ok and safe, we might want to backport this in
previous versions.

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -90,6 +90,7 @@ var SnippetEditor = Widget.extend({
      * @override
      */
     destroy: function () {
+        this.cleanForSave();
         this._super.apply(this, arguments);
         this.$target.removeData('snippet-editor');
     },
@@ -634,9 +635,7 @@ var SnippetsMenu = Widget.extend({
      */
     cleanForSave: function () {
         this.trigger_up('ready_to_clean_for_save');
-        _.each(this.snippetEditors, function (snippetEditor) {
-            snippetEditor.cleanForSave();
-        });
+        this._destroyEditors();
 
         this.$editable.find('[contentEditable]')
             .removeAttr('contentEditable')
@@ -817,6 +816,14 @@ var SnippetsMenu = Widget.extend({
         }
         return $.when();
     },
+    /**
+     * @private
+     */
+    _destroyEditors: function () {
+        _.each(this.snippetEditors, function (snippetEditor) {
+            snippetEditor.destroy();
+        });
+    },
     /**
      * Updates the cover dimensions of the current snippet editor.
      *
@@ -1355,15 +1362,11 @@ var SnippetsMenu = Widget.extend({
     /**
      * Called when a snippet has moved in the page.
      *
-     * @todo technically, as a snippet has been moved, all editors should be
-     * destroyed as their snippet options may not correspond to their
-     * selector anymore. However this should rarely (maybe never ?) be the case,
-     * so we might not want to do this as it would slow the editor.
-     *
      * @private
      * @param {OdooEvent} ev
      */
     _onDragAndDropStop: function (ev) {
+        this._destroyEditors();
         this._activateSnippet(ev.data.$snippet);
     },
     /**