Merge escape and URL filters into one

This is needed to avoid the URL filter detecting wrong link boundaries
as the escape filter would change eg. "<http://kde.org/>" target="_blank">http://kde.org/>;" to
<http://kde.org/>" target="_blank">http://kde.org/>; and then the URL filter would detect
"http://kde.org/&gt" as the actual link.

Switching the order wouldn't help because the escape filter would escape
the newly inserted <a href...> links and the user would
get <a href.../a> and no clickable links.

Therefore we first detect the links, replace them with placeholders,
then escape everything, then replace placeholders with actual links.

Reviewed-by: David Edmundson

FIXED-IN: 0.8.0