gst-launch: Use g_unix_signal_add() to handle keyboard interruption

gst-launch: Use g_unix_signal_add() to handle keyboard interruption

Current implementation uses a traditional signal handler and a 250ms
timeout callback in the event loop.  Adding a GSource with
g_unix_signal_add() to the GMainLoop is a much more elegant solution.
The signal handler with this approach can send a message to the bus
directly rather than set a flag as all dispatching intricacies are handled
by GLib.

https://bugzilla.gnome.org/show_bug.cgi?id=693481