ext/flac/: Port flactag to 0.10, add documentation for it and clean it up a bit.

ext/flac/: Port flactag to 0.10, add documentation for it and clean it up a bit.

Original commit message from CVS:
Based on a patch by: Jonathan Matthew <notverysmart at gmail dot com>
* ext/flac/Makefile.am:
* ext/flac/gstflac.c: (plugin_init):
* ext/flac/gstflactag.c: (gst_flac_tag_setup_interfaces),
(gst_flac_tag_base_init), (gst_flac_tag_class_init),
(gst_flac_tag_dispose), (gst_flac_tag_init),
(gst_flac_tag_sink_setcaps), (gst_flac_tag_chain),
(gst_flac_tag_change_state):
* ext/flac/gstflactag.h:
Port flactag to 0.10, add documentation for it and clean it up a bit.
Fixes bug #413841.
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
* docs/plugins/gst-plugins-good-plugins.hierarchy:
* docs/plugins/gst-plugins-good-plugins.interfaces:
* docs/plugins/gst-plugins-good-plugins.prerequisites:
* docs/plugins/inspect/plugin-flac.xml:
* ext/flac/gstflacdec.c: (gst_flac_dec_base_init):
* ext/flac/gstflacdec.h:
* ext/flac/gstflacenc.c: (gst_flac_enc_base_init):
* ext/flac/gstflacenc.h:
Add flactag and flacenc to the documentation and mark
the private parts of the flacdec instance structure as private.
Also use gst_element_class_set_details_simple() in flacdec and
flacenc.