Dialog: Apply overflow: hidden to contain the resize handles

Fixes #9521
Closes jquerygh-1092

Author: scottgonzalez

themes/base/jquery.ui.dialog.css

@@ -9,6 +9,7 @@
  * http://api.jqueryui.com/dialog/#theming
  */
 .ui-dialog {
+	overflow: hidden;
 	position: absolute;
 	top: 0;
 	left: 0;