Changelog:
- made displays also immune to retracting sticky pistons (not just pistons)
- fixed that new displays would overwrite the data of previous placed ones if a display is destroyed which was placed even before that
- changed how displays are saved
- added a system to convert old displays to the new system, so you don't need to worry about it

The blocks of the item display are now undestroyable by player interactions and explosions and immovable by pistons. The item is prevented from being picked up by entities just as by hopper minecarts. I hope I didn't miss anything.