Item spawn enhancement

The new item spawn system does not work inside of plain items groups, but it should, and does with this PR.

The second commit is completely optional, it adds random charges to sand that spawns in fields (group "field") and to matches that spawn from the group "trash".
DEBUG: Error loading data from json: data/json//item_groups.json: line 51:7: tried to start array, but found '{', not '['