Add API for hidden webview widget creation

[aidanholm]

E.g. webview.new{non_ui = true}.

The purpose of this API is to indicate that a webview is used by a Lua module to perform background webpage loads. This would allow modules to avoid performing unnecessary initialization or unwanted actions like saving history. E.g. modules session, history, formfiller, search, webinspector, and image_css could all be disabled.