v2.7.0 playlist editor enhancements release

NEW this release:

• Added drag-and-drop track reordering facility to the playlist editor #119
• Added automatic source map generation (for developers) #130
• Added JSON playlist import/export facility #126 (from hotfix-2-6-0)

FIXED this release:

• Playlist track timestamp facility broken for shows which span midnight #131
• Random freeze of the track add facility #123 (from hotfix-2-6-0)