Show all tracks

>1. If the artist is selected, all tracks are shown with headers indicating the albums.
2. There is an option called `auto_expand_albums`. If it's set to `false`, the player doesn't automatically select the album when commands such as `win-sel-current` are executed and instead selects the artist. If the albums of this artist are already expanded, the album is selected.

I think the code is quite good now. Please take a look.