Finish moving HTTP_STATUS_* constants to main-response.php
First draft of HTTP response classes

Expand QlResponse, add QlResponseEntity (mostly an interface) and
QlXhtmlMinResponseDocument, which allows to generate XHTML documents and
send them to the remote client via a QlResponse instance.
More code style conversion to always use { and }

Continue what commit 8caae3f672b20b407eac2e904471b0940358e701 started.

Also add semicolons to SQL statements.