Add Variable.watcher, List.watcher attributes
scratch20: Load lists
Refactor scratch14 list saving to use List.watcher
Volume shouldn't be Project global
Block.translate() raises error for unsupported plugin, and a coupla fixes
scratch20: Load variable & block watchers
Raise error when TranslatedBlockType can't match command
Improve block repr
Update todo
images.py: fix so every image is converted, not "every other"
Cleanup blocks2raw a little
Add Watcher.kind, Watcher.value
scratch20: save variable & block watchers
scratch20: save variables
scratch20: save list watchers
Refactor scratch20
scratch20: script, sprite counts
Fix Block number insert casting bug
scratch20: load/save simple attributes (eg. direction).
Add Sprite.size, Sprite.is_visible; Rename Watcher.{ -> is_}visible