MDL-34328: block dragdrop - disallow dragging from show/hide and dock icons

Tell the YUI drag/drop delegate that the show/hide and dock icons are not valid drag handles.  Otherwise, a drag can be initiated by showing or hiding a block (probably not from docking one, but better safe than sorry).  Not strictly related to MDL-34328, but likely to be encountered whilst testing - and closely related to another patch for MDL-34328.

Note that the "invalid" config option for the delegate takes a string - not an array - but can have multiple selectors separated by commas.

lib/yui/blocks/blocks.js

@@ -74,6 +74,7 @@ YUI.add('moodle-core-blocks', function(Y) {
                     nodes: '.'+CSS.BLOCK,
                     target: true,
                     handles: ['.'+CSS.HEADER],
+                    invalid: '.block-hider-hide, .block-hider-show, .moveto',
                     dragConfig: {groups: this.groups}
                 });
                 del.dd.plug(Y.Plugin.DDProxy, {