Support log domain specific log handlers

[wouter bolsterlee]

It would be useful to complement the AnewtLog::add_handler() API with AnewtLog::add_handler_for_domain(). The log handlers added with the latter will only log messages with the specified domain.

That being done, Anewt modules can start logging all kind of useful stuff using the 'anewt' domain. An example use case debug messages with the executed SQL queries in the database layer.

[wouter bolsterlee (wbolster)]

It could also be useful to have the log domain stuff handle everything starting with a prefix, so that e.g. database queries can be logged using the 'anewt-database' log domain.

Perhaps some convention like AnewtLog::add_handler_for_domain('anewt*', $handler); can be used for this.