Playlist Save Preferences 2

I hope that this is a better solution for https://github.com/clementine-player/Clementine/issues/4480 than both https://github.com/clementine-player/Clementine/pull/4481 and https://github.com/clementine-player/Clementine/pull/4484. It puts the path preferences in the preferences, but also allows the user to show a "Quick Change" menu at the top of the playlist. (See images below.)
![prefsdialog](https://cloud.githubusercontent.com/assets/3172687/3940479/70a47218-24e2-11e4-874a-23461a44fcd1.png)
![playlistcontainer](https://cloud.githubusercontent.com/assets/3172687/3940480/79512604-24e2-11e4-9820-45add440f304.png)

It was a good idea to have constant for this :) Playlist::kWriteMetadata
Why not reusing it here?