Add apev2mux element (#343122).

Add apev2mux element (#343122).

Original commit message from CVS:
Patch by: Sebastian Dröge  <mail at slomosnail de >
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
* ext/taglib/Makefile.am:
* ext/taglib/gstapev2mux.cc:
* ext/taglib/gstapev2mux.h:
* ext/taglib/gstid3v2mux.cc:
* gst-libs/gst/tag/gsttagmux.c: (plugin_init):
* gst-libs/gst/tag/gsttagmux.h:
Add apev2mux element (#343122).
* tests/check/Makefile.am:
* tests/check/elements/apev2mux.c:
(test_taglib_apev2mux_create_tags),
(test_taglib_apev2mux_check_tags), (fill_mp3_buffer), (got_buffer),
(demux_pad_added), (test_taglib_apev2mux_check_output_buffer),
(test_taglib_apev2mux_with_tags), (GST_START_TEST),
(apev2mux_suite), (main):
Add unit test for apev2mux element.