Use Variant.check_format_string() for type check

The types are not necessary same, but are compatible.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
MessageHandler's response as return value

Use return value for MessageHandler's response instead of out arguments.
Handlers can be written more naturally.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>