Introduce new source discoverer

The new source discoverer is a complete overhaul of
the current code. It is async-only, correctly threaded,
and easier to maintain.

A major new feature of this web source discoverer is
that it now pings the server to see if the URL feeds
us back with an iCalendar file.

Furthermore, it continues to use E-D-S WebDAV discovery
functions as a fallback mechanism.

Fixes #380

tests/gcal-simple-server.c

@@ -88,7 +88,7 @@ static void
 process_caldav (SoupMessage *message,
                 const gchar *path)
 {
-  g_message ("Processing CalDAV");
+  g_debug ("Processing CalDAV request");
 }