The actions to execute after a content change were factorized in
separate functions (vlc_playlist_ItemsInserted(), etc.).

These functions also updated the current media on the player. Instead,
update it separately, so that we can make several changes while
updating/invalidating the current media only once.

This paves the way to improve the implementation of
vlc_playlist_Expand().

Signed-off-by: Thomas Guillem <thomas@gllm.fr>