gsettings: stop using g_settings_list_schemas()

Stop using the old deprecated trick of listing and iterating over all
the schemas.  Use g_settings_schema_source_lookup() instead.

https://bugzilla.gnome.org/show_bug.cgi?id=720232
Add some G_GNUC_PRINTF annotations

Fix clang build with -Werror=format-nonliteral.

https://bugzilla.gnome.org/show_bug.cgi?id=720231