Fix playlist "Insert" a bit.

Fix playlist "Insert" a bit.
If insert was used to create a new playlist (e.g. inserting without audio playing), the insert position wasn't updated. Thus, the playlist created was in reversed order (except for the first song).
This was the reason "Insert last" was actually used as Insert in this case. BUT, this one doesn't update the insert position eiterh, but the order of the songs is fine, at least (due to appending).
Now, insert always updates the insert position properly, so that the aforementioned bugs are fixed. Insert is always used now.
Other impact: If you create a playlist using insert, every subsequent insert will append songs at the end of the playlist,if you start playback pressing select on songs, every subsequent insert will insert after the current track.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@20241 a1c6a512-1295-4272-9138-f99709370657