added support for URL action and alert channel (incl. HTTP)

Redesigned UDP action and alert channel into more general URL
request action and alert channel which support both UDP and QNetworkRequest
(hence HTTP, FTP, file://).

This is available through this action:
  (requesturl (address udp://localhost:8125) "foo.bar.%!taskid:1|c")
  (requesturl (address http://localhost/foo)(param method delete))
And this alert channel:
  (url (address udp://localhost:8125) "warning: %:alertid")
  (url (address "http://localhost/foo?warning=%:alertid"))

Other protocols (ftp, file) are likely to work but not tested.

Also factorized network code between UDP action and UDP alert channel,
and between HTTP exec mean and QNetworkRequest action and alert channel.