Clean Up Database Signal Interface

[mvandeberg]

To implement #1996, #2149 changed the interface for plugins registering with a signal to save the name for statistics collecting.

Each plugin has a name() method already, that should be used here. Instead of creating a proxy method, which isn't well documented and confusing when the signal is still exposed, I suggest creating a macro PLUGIN_REGISTER_SIGNAL( signal, method ) that calls the appropriate method in the database, creates the lamba properly, etc. This will also allow use of name(), so the correct name is always registered and is a much cleaner interface.

The group can also be exposed by the macro. It should still return the connection object to be stored in the plugin itself.