Overhaul a little the QUrl error reporting.

Overhaul a little the QUrl error reporting.

Keep the original QString that triggered the parsing error, instead of
just one QChar. This provides more powerful error messages, like:

 Invalid IPv6 address; source was "http://[:::]";  scheme = "http", host = ""
 (QUrl cannot keep invalid hostnames)

 Invalid port or port number out of range; source was "http://example.com:abc";  scheme = "http", host = "example.com"
 (QUrl cannot keep a non-numeric port number)

 Invalid path (character '%' not permitted); source was "foo:/path%?";  scheme = "foo", path = "/path%25%1F"
 (the tolerant parser runs first, so the faulty component is fixed)

This stores the error state in a special structure which is not
allocated under normal conditions, keeping the memory consumption
down. On 32-bit systems, QUrlPrivate does not increase in size; on
64-bit systems, it grows by 8 bytes.

Change-Id: I93d798d43401dfeb9fca7b6eed7ea758da10136b
Reviewed-by: David Faure <faure@kde.org>