Sprite management

[dekrain]

ToDo list:

• Create Sprite button
• Remove Sprite button
• Rename Sprite button

[thisandagain]

Thanks for the PR. At this time we'd like to hold off on adding any new GUI features to the VM playground.

We are in the middle of getting the actual Scratch 3.0 editor interface together and hope to open source that repo over the next several weeks. Once that repo is available features like being able to add, remove, and rename sprites would be most welcome over there. For now, I'm going to close this PR. Many thanks.

/cc @tmickel @rschamp