playlist: do not move both tree and non-tree items + restrict move action

playlist: do not move both tree and non-tree items + restrict move action

Items are stored twice, once in a tree structure and once in a one-level list.
This patch removes attempt of the playlist_TreeMove() function to move both tree and
one-level instances of an item.

Firstly the reason is that function was not effective in this attempt.
And secondly, the attempt itself doesn't make sense in some cases: you can always
map moves within the tree to moves within one-level, but you can't always do it
the other way around, it is in most cases ambiguous.

Moreover, this patch restricts moves in the tree to within the present parent of the
item being moved. Again, in most cases, moving an item out of its parent or into another
parent is meaningless.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>