GSettings: properly support 'extends'

Support the 'extends' attribute that has been supported by the compiler
for a long time by doing three things:

 - when creating a schema that extends another schema, lookup that other
   schema

 - when looking up keys and we can't find them in the schema, check
   (recursively) in the 'extends' schema

 - when listing all keys in a schema, also visit the extends schemas,
   but take care to avoid duplicates caused by overrides

Extend the testsuite to verify that it works.

https://bugzilla.gnome.org/show_bug.cgi?id=645453