Use Caja GSettings schema optionally

This makes Caja dependency optional.
g_settings_schema_source_lookup returns a GSettingsSchema that should be freed
See: 
https://developer.gnome.org/gio/2.32/gio-GSettingsSchema-GSettingsSchemaSource.html#g-settings-schema-source-lookup
http://git.mate-desktop.org/mate-desktop/tree/libmate-desktop/mate-gsettings.c#n44