Edit/Create Sequence of song parts using drag&drop of part codes

This way avoiding the existence of invalid song-part codes in the sequence

[x] 01 create draggable items for each existing song part and after adding a new one
[x] 02 create drop zone to clone the items into and bin zone in order to have them removed when needed
[x] 03 adapt deletion and adding methods to remove or add droppable items
[x] 04 write the existing sequence into the drop zone
[x] 05 Make sure the size of the drop-zone is always big enough for another "drop"
[x] 06 add macro to create the default sequence from existing song parts
[x] 07 add macro to handle the dropping and to write the new sequence into the DOM and the backend
[x] 08 make drop-zone items sortable, so that the sequence can be re-arranged
[x] 09 What happens when the existing sequence becomes invalid due to a deleted or added song part?
[x] 10 Prevent deletion of a song part as long as it is still part of the sequence
[x] 11 Add help text to Sequence area: "You can't delete a song part while it's still assigned to the sequence".
[x] Layout needs significant improvement
[x] Not showing "Save" button after only re-arranging the sequence

matthiku / cSpot