gst/spectrum/: port to use message to get results, cleanly exit when closing the...

gst/spectrum/: port to use message to get results, cleanly exit when closing the window

Original commit message from CVS:
* gst/spectrum/demo-audiotest.c: (on_window_destroy),
(draw_spectrum), (message_handler), (main):
* gst/spectrum/demo-osssrc.c: (on_window_destroy), (draw_spectrum),
(message_handler), (main):
port to use message to get results, cleanly exit when closing the window
* gst/spectrum/gstspectrum.c: (gst_spectrum_class_init),
(gst_spectrum_init), (gst_spectrum_dispose),
(gst_spectrum_set_property), (gst_spectrum_get_property),
(gst_spectrum_set_caps), (gst_spectrum_start),
(gst_spectrum_message_new), (gst_spectrum_transform_ip):
* gst/spectrum/gstspectrum.h:
port to derive from basetransform and send results via messages
(like level element)