Take this private API and make it public along with a boxed type and
ref/unref functions.
Future commits will add accessors with new functionality and some that
allow us to deprecate functions on GSettings itself (such as
g_settings_get_range).
https://bugzilla.gnome.org/show_bug.cgi?id=668232
These functions no longer have anything to do with GSettings itself, so
they should not be in that file anymore.
GSettings still wants direct access to the GSettingsSchemaKey structure,
so put that one in gsettingsschema-internal.h.