As it was preventing the custom URI handlers to be called.

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 ().

We have to remove HTML meta tag with charset property set from
text/html parts of messages. Before we load message into web view Evolution
is transforming all messages to UTF-8. When WebKit2 loads the message that is
transformed by Evolution to UTF-8, but the message contains meta tag
that specifies the encoding WK2 prefers the encoding given by that meta
tag. So we have to remove this meta tag to show the message with right
encoding.

The background color class was not set, so it was acting weirdly. Also
the left margin for left button was set to zero to align it with iframe
below.