Sort out class construction

[Chris Coulson]

As QML doesn't have any concept of construct parameters (any property can be set whilst an object is being constructed) and Oxide needs construct parameters in some cases (eg, WebView.context because we need a BrowserContext to construct WebContents, and WebContext.dataPath because we need a data path before we can construct a BrowserContext), we currently have a fair bit of code that handles this oddity in Oxide.

However, this code is scattered around various places - eg, for WebView, it's in oxide::WebView and OxideQQuickWebView. For WebContext, it's in oxide::qt::WebContext. For UserScript, it's in OxideQQuickUserScriptPrivate. Given that oxide::qt::WebView no longer subclasses oxide::WebView (it's a client of it), it should be easier to refactor this code all in to the same layer.

As this is a limitation of QML, ideally this code would be refactored in to the classes in qt/quick/