Load and cache favicons with libsoup instead of GIO

Load and cache favicons with libsoup instead of GIO

The previous code relied on GIO implementing HTTP
with libsoup, so this is effectively not a new
dependency. And since many people don't have GVfs
and even if they do, somehow GIO doesn't work as
good as one should expect, direct use of libsoup
is actually an improvement. Plus the new code
caches icons on disk which we didn't do before.