Java bindings property notifications

[Rugvip]

I'm not sure how to solve signal disconnection, but the only two alternatives that I see are: return a handler object with a disconnect() method, or have add a removeChangeListener method and figure out a way to disconnect the correct handler. Either way the API won't change, so this should be fine to add as is.

[sdroege]

I would just connect the native listener dispatcher to the signal during instantiation of the managed object, and disconnect it when the managed object is finalized.

[sdroege]

Alternatively allow setting a null listener (i.e. unsetting of the listener), and there disconnect.

[sdroege]

g_signal_handlers_disconnect_by_func() might also be useful if you don't want to keep the signal handler ids from g_signal_connect() around

[Rugvip]

So i guess the conclusion from the discussion on IRC is that both addListener/removeListener and setListener are possible to implement using hashtables in Java or GObject data, just need to decide which one we want.

[superdump]

Any pros/cons?

[Rugvip]

I went with the add/remove approach, although in a separate commit. I'll probably close this PR.

[Rugvip]

This was implemented in #114 using add/remove approach.