Logging improvements added to XML-RPC

It was changed how response is logged so that thrown expception
doesn't prevent debugging message with the whole response to be
logged.

New logger was also added to provide parser logging.  It is extremely
verbose even on info level, but it's a parser and XML is verbose on
its own.
Improved XML-RPC response parsed

It can newly handle empty value elements.
Subscription dialog behavior extended

Subscribe button is disabled until some calendar is selected.  New
dialog status delegate was create to make this possible.