Epiphany was registering its "ephy-about" handler by directly calling
soup_session_feature_add_feature() on the SoupRequester, rather than
calling soup_session_add_feature_by_type() on the session. Fix it so
that will still work.

Add soup_session_request() and soup_session_request_uri(),
implementing basically the same behavior as soup_requester_request()
and soup_requester_request_uri() (but without requiring a separate
SoupSessionFeature), and remove the unstable-api ifdefs from
soup-request*.h.

SoupRequester still exists, but it is still guarded by the
unstable-api ifdefs, and is just a dummy wrapper around the
functionality that is now in SoupSession.

Especially, include soup.h rather than individually including a bunch
of other public soup-*.h files.

Remove unnecessary system includes (many are leftovers from code that
has moved down into glib).

Also, prefix virtual method implementation names with the class name,
to be consistent with other code.

There were already error messages in soup-request.c and
soup-requester.c marked for translation, and we'll be adding more
soon.

WebKit already exposes the SoupSession to its users, so this change
will automatically give them access to the SoupRequest infrastructure
as well.

SoupRequest stuff is based on Gabriel Corvalan's Google Summer of Code
project from 2009, as further hacked on at the 2009 WebKitGTK Hackfest.

The cache stuff was written on top of that by Sergio Villar and
initially committed to WebKit since it only works with the SoupRequest
infrastructure, and the SoupRequest stuff was never fully completed
(eg, the synchronous versions never worked right).

Move all of this here now as a first step to finishing it, but hide it
behind #ifdef LIBSOUP_USE_UNSTABLE_REQUEST_API.