fix #16482 Regression: _WidgetBase#placeAt() can't be used on a
DocumentFragment

* added tests for placement using document fragment
* updated WidgetBase.placeAt to allow a document fragment
Limit background iframe to IE; it's no longer needed on other browsers.
Also switch test from java applet to PDF, since the java applet wasn't loading correctly.
Fixes #17811, refs #11822.