Add API for making a GStreamer plugin 'dependent' on external files, directories...

Add API for making a GStreamer plugin 'dependent' on external files, directories or environment variables, so that GS...

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt::
* gst/gst_private.h: (GstPluginDep), (_GstPluginPrivate):
* gst/gstplugin.c: (gst_plugin_init), (gst_plugin_finalize),
(gst_plugin_class_init), (gst_plugin_list_free),
(gst_plugin_ext_dep_get_env_vars_hash),
(_priv_plugin_deps_env_vars_changed),
(gst_plugin_ext_dep_extract_env_vars_paths),
(gst_plugin_ext_dep_get_hash_from_stat_entry),
(gst_plugin_ext_dep_direntry_matches),
(gst_plugin_ext_dep_scan_dir_and_match_names),
(gst_plugin_ext_dep_scan_path_with_filenames),
(gst_plugin_ext_dep_get_stat_hash),
(_priv_plugin_deps_files_changed), (gst_plugin_ext_dep_free),
(gst_plugin_ext_dep_strv_equal), (gst_plugin_ext_dep_equals),
(gst_plugin_add_dependency), (gst_plugin_add_dependency_simple):
* gst/gstplugin.h: (GstPluginPrivate), (GstPluginFlags),
(GST_PLUGIN_DEPENDENCY_FLAG_NONE),
(GST_PLUGIN_DEPENDENCY_FLAG_RECURSE),
(GST_PLUGIN_DEPENDENCY_FLAG_PATHS_ARE_DEFAULT_ONLY),
(GST_PLUGIN_DEPENDENCY_FLAG_FILE_NAME_IS_SUFFIX),
(GstPluginDependencyFlags), (GstPluginFilter):
* gst/gstregistry.c: (gst_registry_scan_path_level):
* gst/gstregistrybinary.c: (gst_registry_binary_save_feature),
(gst_registry_binary_save_plugin_dep),
(gst_registry_binary_save_plugin),
(gst_registry_binary_load_feature),
(gst_registry_binary_load_plugin_dep_strv),
(gst_registry_binary_load_plugin_dep),
(gst_registry_binary_load_plugin):
* gst/gstregistrybinary.h: (GST_MAGIC_BINARY_VERSION_STR),
(GstBinaryPluginElement), (_GstBinaryDep), (GstBinaryDep):
* gst/gstregistryxml.c: (gst_registry_xml_save_plugin):
Add API for making a GStreamer plugin 'dependent' on external files,
directories or environment variables, so that GStreamer knows when
it needs to re-load GStreamer plugins that wrap other plugin systems.
Fixes bug #350477.
API: add gst_plugin_add_dependency()
API: add gst_plugin_add_dependency_simple()