Bring Ext.ux.RapidApp.showIframeWindow into the fold with newer APIs

[vanstyn]

Ext.ux.RapidApp.showIframeWindow is the very old, before the Template::Controller stuff, and needs brought into the fold with the newer stuff (or unfactored). It is only used in one place, the HtmlEditor "Load & Download HowTo" page which also very old and has dead links, etc

However, some of this stuff was interesting and may be useful, so it might not necessarily be discarded out of hand...