Add the GLIB_AVAILABLE_IN_ALL annotation to all old functions (that
haven't already been annotated with the GLIB_AVAILABLE_IN_* macros or a
deprecation macro).
If we discover in the future that we cannot use only one macro on
Windows, it will be an easy sed patch to fix that.
https://bugzilla.gnome.org/show_bug.cgi?id=688681
Since #include <gsettingsbackend.h> is a perfectly valid thing for
applications to do, and since we want to include gio headers from
gsettingsbackend.h, we need to effectively disable the #error we would
get from those headers (because we're not coming via gio.h).
We don't want to #include <gio/gio.h> here because this would cause
needless rebuilding of GSettingsBackend, GSettings,
GDelayedSettingsBackend, etc... every time someone changed anything in
any public header.
