Use it before loading projects so targets don't accumulate when multiple projects are loaded on the same instance.
Move check to see if the clone is the original clone onto the block implementation so all clones can be removed.
Fixes#274
* 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)
