Polish the quickplayer example.

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

Task-number: QTBUG-36730
Task-number: QTBUG-37203
Change-Id: I7a133283c0d4acd521957a05066f9bb7f9d98b70
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>