Dialog: Set top and left properties in the default styles to avoid a …

…bug in core. Fixes #8741 - Dialog: dialog with position:fixed goes offscreen when opening after scrolling down since v 1.9.0.

themes/base/jquery.ui.dialog.css

@@ -8,7 +8,7 @@
  *
  * http://docs.jquery.com/UI/Dialog#theming
  */
-.ui-dialog { position: absolute; padding: .2em; width: 300px; overflow: hidden; }
+.ui-dialog { position: absolute; top: 0; left: 0; padding: .2em; width: 300px; overflow: hidden; }
 .ui-dialog .ui-dialog-titlebar { padding: .4em 1em; position: relative;  }
 .ui-dialog .ui-dialog-title { float: left; margin: .1em 16px .1em 0; }
 .ui-dialog .ui-dialog-titlebar-close { position: absolute; right: .3em; top: 50%; width: 19px; margin: -10px 0 0 0; padding: 1px; height: 18px; }