Deduplicate debug server connection code.

Deduplicate debug server connection code.

The packet protocol can be part of the server, now that the server is
not part of QtQml anymore. This enables us to remove some duplicated
code from the connections.

As an added benefit, with more control over the sending process,
QQmlDebugServer can now efficiently send single packets, without
creating a QList<QByteArray> first.

Change-Id: I13cc831e254c02b737e64816d6d3ab051d760995
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>