Better string needed for Amarok's unified view mode

[Bart Cerneels]

We have this string in the collection browser: "Toggle unified view
mode" It's to enable the view that contains tracks from all
collections. We used to call this "Flat View" I think.

Right now I'm adding something similar to the Podcast category and
later I'll do the same with the Saved Playlists.

I think we need a better string:
- For one if we use QAction::setCheckable( true ) we get checkbox for
free, scratch the "Toggle"
- second: "unified view mode" sounds complicated. We should be able to
find something most people will understand instantly.

Bart