convenience: use recursive lookup for schemas

If the schema is not found in the extension directory, fall back
to the default source, allowing for systemwide installation of
extensions.

lib/convenience.js

@@ -58,7 +58,7 @@ function getSettings(schema) {
     else
         schemaSource = GioSSS.get_default();
 
-    let schemaObj = schemaSource.lookup(schema, false);
+    let schemaObj = schemaSource.lookup(schema, true);
     if (!schemaObj)
         throw new Error('Schema ' + schema + ' could not be found for extension '
                         + extension.metadata.uuid + '. Please check your installation.');