MessageEngine debug mode

[acasteigts]

The DefaultMessageEngine offers a method setDebug() that makes it possible to print a log of the messages in the Standard Error. It would be good to generalize this method into the MessageEngine interface, so that all message engines can offer such a feature, or to externalize it to another component through the message listeners interface.

[acasteigts]

A method setDebug() was added to the MessageEngine interface. A default (empty) implementation was provided, for compatibility with potential existing message engine.

Merged into develop.

[acasteigts]

(Will be closed when released.)

[acasteigts]

The above changes correspond to commit 6e499fa (the commit message was missing reference to this issue).

[acasteigts]

Merged into develop.