Improved shuffle mode

When a track is manually chosen in shuffle mode, it is inserted at current
position in the shuffle list.
Rename several functions

  tree_set_selected ->   tree_activate_selected
sorted_set_selected -> sorted_activate_selected
    pl_set_selected ->     pl_activate_selected

      tree_set_next ->   tree_goto_next
    sorted_set_next -> sorted_goto_next
        pl_set_next ->     pl_goto_next

      tree_set_prev ->   tree_goto_prev
    sorted_set_prev -> sorted_goto_prev
        pl_set_prev ->     pl_goto_prev
Merge pull request #87 from mahkoh/shuffle

Shuffle + Rename

Should make #39 a bit better.