Add GSettings support (GConf is going away).

Add GSettings support (GConf is going away).

* configure.in (gsettings): New option and check for GSettings.

* src/Makefile.in (SETTINGS_CFLAGS, SETTINGS_LIBS): Renamed from
GCONF_CFLAGS/LIBS.

* src/xsettings.c: Include glib.h if HAVE_GSETTINGS.
Let HAVE_GSETTINGS override HAVE_GCONF.
(store_monospaced_changed): New function.
(EMACS_SETTINGS): A new type derived from GObject to handle
GSettings notifications.
(emacs_settings_constructor, emacs_settings_get_property)
(emacs_settings_set_property, emacs_settings_class_init):
New functions.
(gsettings_client, gsettings_obj): New variables.
(GSETTINGS_SCHEMA): New define.
(something_changedCB): Call store_monospaced_changed.
(init_gsettings): New function.
(xsettings_initialize): Call init_gsettings.
(syms_of_xsettings): Initialize gsettings_client, gsettings_obj
to NULL.