scratch20: load costumes
Add Project.copy() for deep-copy
Conversion interface. Warn for SVGs and output placeholder. Add Image.new.
Friendlier block conversion errors
Fix Block.copy()
Workaround BlockTypes with different arguments: 'stop' and 'forever if'
Tweak block errors
Fix Script.copy
Fix Image.new parameter ordering
Clean up BlockType
Custom blocks (scratch20)
Allow CustomBlockTypes as argument to Block
Fix stringify() empty boolean inserts
Reorder submodule imports
Don't warn for ambigious arg to BlockType.get when unnecessary