ext/wavpack/: Use GSlice for allocating index entries and use gst_element_class_set_details_simple().

Original commit message from CVS:
* ext/wavpack/gstwavpackdec.c: (gst_wavpack_dec_base_init):
* ext/wavpack/gstwavpackenc.c: (gst_wavpack_enc_base_init):
* ext/wavpack/gstwavpackparse.c:
(gst_wavpack_parse_index_entry_new),
(gst_wavpack_parse_index_entry_free),
(gst_wavpack_parse_base_init),
(gst_wavpack_parse_index_append_entry), (gst_wavpack_parse_reset):
Use GSlice for allocating index entries and use
gst_element_class_set_details_simple().