MINOR update formfield when the subsite is changed (from r91016)

javascript/LeftAndMain_Subsites.js

@@ -1,6 +1,7 @@
 Behaviour.register({
 	'#SubsiteActions select' : {
 		onchange: function() {
+			$('Form_AddPageOptionsForm_SubsiteID').value = this.value;
 			var request = new Ajax.Request(SiteTreeHandlers.controller_url + '/changesubsite?SubsiteID=' + this.value + '&ajax=1', {
 				onSuccess: function(response) {
 					if ($('sitetree')) {