Connect signals from Builder objects

[jwharm]

It would be nice to connect signals from Builder objects, like this:

var button = Button.builder()
    .setLabel("Close")
    .onClicked(window::close)
    .build();

This can be implemented in the base Builder class with a HashMap of signal names and callbacks, and after the new GObject has been constructed, it would iterate through the map and connect the signals to the callbacks.

Depends on #125 to be fixed first, because currently the SignalHandler object that is returned by the connect method (such as onClicked above) is the only available mechanism to disconnect the signal and free the allocated memory.