* Import lists and variables from SB2
* Switch to Variable and List objects
* Add Clone.lookupOrCreateVariable, Clone.getVariable, Clone.setVariable
* Add (get, set, change) variable blocks.
* Copy variables and lists on clone instantiation
* Move variable options closer to blocks
* Add list primitives
* Move variable and lists storage to `Target` instead of `Clone`
* Move _computeIndex to a Cast function
* Rename `getList` -> `getListAsString`
* Renames renames
* Remove extra check in Cast.isNaN
* Provide property to Clone to distinguish "original" clones
* Provide method to clone a clone's properties
* Don't report clones in the UI target list
* Add target info to Thread
* Allow hats to skip clones (for green flag)
* Green flag skips clones
* Implement "create clone" and hat
* Pass the runtime to sprites and clones (for start hats)
* Clone disposal; trigger hats after drawable initializes.
* Separate stop threads for target; fix handling of stop button
* Remove extraneous `skipClones` property
* Add global clone limit
* Don't allow a non-clone to delete itself.
* Rename `cloneClone` -> `makeClone`
* Variable updates in runtime.js
* Synchronous drawable initialization (until we put it back to promises)
