Tell loader how to use WebVfx.

With my latest (merged) patches to WebVfx, one can load plain old HTML
and QML files. They do not need webvfx script to initialize rendering -
with some caveats about the meaning of document-loaded. However, without
this loader change, plain resources need  to prefaced with "plain:". The
loader producer can now do that and instead "webvfx:" is needed to tell
webvfx to wait for script in the content to call
WebVfx.renderReady(true) (unless you use "webvfx:plain:...").

Also, WebVfx can also now load HTML over HTTP. This is especially handy
if your content is updating itself with data from a web service.
Otherwise, you will run into cross-site-scripting errors.