Port to new Diorite.Ipc.MessageHandler signature

 -  It is delegate with a target and throws an exception instead of
    false return value.

 - Throw exceptions instead of create_error() helper func (is gone).

 - Use the new check_type_str() helper func.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
WebEngine: Don't use idle callback to show errors

It was necessary when messages were received in another thread.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Use async IPC server in WebExtension

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