Initialize WebKit extension in the callback on the initialize-web-extensions signal from WebKitWebContext.

Register custom uri schemes handlers in the previously mentioned
callback. Also add the register-uri-handlers signal into the EWebView
to allow registering the custom uri scheme for its descendants and use
it in EMailDisplay.

Don't pass WebView into the custom uri handlers, but use the web view
returned by webkit_uri_scheme_request_get_web_view ().