make lyrics cached.
this sets the HTML of the lyrics to be cached.

reasoning: previously, if you had a track playing and resume on start enabled, and had the lyrics applet visible, when opening amarok you would have to press the refresh button to see the lyrics (this is because the scriptmanager *must* be created after MainWindow).
so caching fixes this bug

if you notice *any* lyrics weirdness, please get a hold of me.