BarcodeReaderGTK: Emit a GObject signal on barcode

This is probably more GNOMEy.
I was wondering how to emit a signal with the GStreamer message as
second argument. What type would I use when registering the signal?
Connect to the barcode signal

also, a very early Key class was introduced. I'd be surprised if it
survives for long as it may not be very useful and has probably been
implemented multiple times already. Monkeysign must use something
similar.
Replaced the Save button with a Load button

Eventually, we may want to load an image. While it's not entirely clear
how we would design that, this could serve as a basis.