Add method for getting domain from PluginInfo.

Plugins shouldn't be required to have getters and setters for
something that's constant.

Change-Id: I99ebc7d10f558008a7889252cc3c96f39747303e
Reviewed-on: http://gerrit.dmdirc.com/2860
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>