configure.ac: Add DATADIR for storing presets.

configure.ac: Add DATADIR for storing presets.

Original commit message from CVS:
Patch by: Stefan Kost  <ensonic@users.sf.net>
* configure.ac:
Add DATADIR for storing presets.
* docs/gst/gstreamer-docs.sgml:
* docs/gst/gstreamer-sections.txt:
* docs/gst/gstreamer.types.in:
Add GstPreset to docs.
* gst/Makefile.am:
* gst/gst.h:
* gst/gstpreset.c: (preset_get_paths), (preset_skip_property),
(preset_open_and_parse_header), (preset_parse_version),
(preset_merge), (preset_get_keyfile),
(gst_preset_default_get_preset_names),
(gst_preset_default_get_property_names),
(gst_preset_default_load_preset),
(gst_preset_default_save_presets_file),
(gst_preset_default_save_preset),
(gst_preset_default_rename_preset),
(gst_preset_default_delete_preset), (gst_preset_default_set_meta),
(gst_preset_default_get_meta), (gst_preset_default_randomize),
(gst_preset_default_reset), (gst_preset_get_preset_names),
(gst_preset_get_property_names), (gst_preset_load_preset),
(gst_preset_save_preset), (gst_preset_rename_preset),
(gst_preset_delete_preset), (gst_preset_set_meta),
(gst_preset_get_meta), (gst_preset_class_init),
(gst_preset_base_init), (gst_preset_get_type):
* gst/gstpreset.h:
Add GstPreset to core. Fixes #396779
* tests/check/Makefile.am:
* tests/check/gst/gstpreset.c: (gst_preset_test_get_property),
(gst_preset_test_set_property), (gst_preset_test_class_init),
(gst_preset_test_base_init), (gst_preset_test_get_type),
(gst_preset_test_plugin_init), (GST_START_TEST),
(remove_preset_file), (test_setup), (test_teardown),
(gst_preset_suite):
Add GstPreset unit tests.