Properly hook grid into model items, partial implementation

Loading items does not associate them with the sheet grid!
This will lead to misplacment after rotation or flipping - for now.
Add item_data_snap, remove dead code chunks
Remove overlapping signal "changed" of wire and item-data
Properly unregister items on removal

Due to a missing unregister call, connection dots would remain.
Also fixes a bunch of warnings and whitespaces errors