Polish the musicplayer example.

Use QStandardPaths::MusicLocation for the file diag; improve
arguments handling.

Task-number: QTBUG-36730
Task-number: QTBUG-37203
Change-Id: Id408ad29e9d7b00eed0b230e43a7232777bb7afd
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>