skins2: fix some important playlist limitations or bugs

skins2: fix some important playlist limitations or bugs

This patch includes the following :
  - fix item misplacement (items were only appended instead of being inserted)
  - fix slider scrolling that could not adapt to the real size of the playlist
  - enhance drag&drop by allowing users to finely insert item being dropped
    into either the playlist or the media library.
  - optimise refresh (only rebuild playtree in case of visible item)
  - remove keeping a reference to a playlist_item_t* (since it is not
    refcounted, a lookup from the playlist with proper lock mechanism
    is needed)
  - remove the m_deleted flag (corner cases were never dealt with) and
    replace it with a notification prior to deletion
  - implement operator++ to simplify iterating visible items (cosmetics)

A deeper redesign/simplification and support for the new sql playlist would be a good thing though.