Receive arbitrary signals

[rm5248]

For a project that I'm working on, I need to have the ability to listen to arbitrary signals on the bus. A few years ago, I patched dbus-java(2.7) to give it this capability. I just tried with the current version of dbus-java and it won't work correctly, due to how signals work(you need to create a class in the correct package for it to be called correctly). Two questions with this:

1. Does this make sense as a feature to add? It's needed for my case, but I don't know if it makes sense to add to the library. Most other dbus implementations do provide a way to listen to arbitrary signals.
2. Assuming it makes sense, the way that I implemented it before was to add in AbstractConnection a new variable for the custom signals, the same type as handledSignals and then looks at these custom handlers after looking at the normal(typed) handlers.

I can do the implementation, I just wanted to get feedback before I started on this.

[hypfvieh]

Sounds good to me. I think this feature maybe useful for other cases as well.
If you do the implementation and send a PR, I'll take a look and merge it afterwards.

[hypfvieh]

Thanks for your PR. I cleaned it up a bit and merged it.

I also removed the sendSignal method, as this method is a copy of sendMessage.
Instead I updated the javadoc of sendMessage (renaming this method would break the compatibility with older code).
I don't think we should add more methods to the AbstractConnection class if not necessary.

[rm5248]

That all looks good to me. I will continue to test it out on my end to ensure that everything is working appropriately.