GSettings: store (default, options) in gvdb

GSettings: store (default, options) in gvdb

gvdb just dropped the ability to have a separate "options" field.  We
now store the options into a GVariant along with the default value.

For now, we use a small shim in GSettingsSchema in order not to touch
too much code.  A more complete rewrite will follow.

This represents a change to the schema file format with another likely
to follow.  glib-compile-schemas needs to be re-run after installing
this change.