Port Diorite.KeyValueStorage subclasses to new API

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Don't allow Nuvola.config.set() to store objects

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Print path of script selected by web app registry

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