Fix the WebKit2 port of Evolution to let it compile
Adapt the code for webkitgtk 2.5.x and require it
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 ().
EWebView, EmailDisplay - Register custom URI schemes in *_class_init functions
EWebView, EMailDisplay - Refactor the context initialization into own function
EWebView - Turn that silly enable-private-browsing option off

As it was preventing the custom URI handlers to be called.
Remove unneeded code after the WebKit context initialization was moved into *_class_init functions