gst/spectrum/gstspectrum.c: Rename property enums and default defines for the properties to match the property names ...

Original commit message from CVS:
* gst/spectrum/gstspectrum.c: (gst_spectrum_class_init),
(gst_spectrum_init), (gst_spectrum_set_property),
(gst_spectrum_get_property), (gst_spectrum_message_new):
Rename property enums and default defines for the properties to match
the property names and rephrase property descriptions to make them a
bit clearer (hopefully). See #518188.