Refactor web cache to circumvent libsoup when needed

Refactor web cache to circumvent libsoup when needed

At this point libsoup is dependent on a functional network adapter,
which may not be available if no network connection is there or
the connection is faulty. So *if* we have a requested page in the
cache, we load it before libsoup comes into play.

Depending on the WebKitGTK+ version, loading from cache uses an
"alternate" URI so that we don't end up having cache filenames
in the location entry.

Generation of cache filenames is fixed to never include special
characters such as slashes or question marks.

Only non-empty files are actually stored to the cache folder.