I implemented some primitives to expose some WebKit user content APIs

(user script and script message handlers) for WebKit Xwidgets, both

for WebKitGTK impl and NS/mac impl.

The user script API makes it possible to reliable and predictable injecting

script into a Webkit Xwidget, which is useful for customizing the WebView

behavior.

The script message handler API makes it possible to trigger event in emacs

from JavaScript, and can be used to implement procedure calling from

js to elisp. Currently only the other way around is possible.

The patch is attached.