Add 'empty' sprite and costume

Check for the selected sound before rendering the editor

Make it so that the user cannot select tabs

Make libraries screen reader accessible.

chore(package): update webpack-dev-server to version 2.9.7

Enable sprite rotation keyboard control and standardize tabbing.

Monitor layout

Reorder menu options so random position is above mouse-pointer

12/6/17 greenkeeper updates to react 16.2 related things

Force 1024x768 window size for automated tests

Try to keep new custom procedure inputs in view

Sloves Tab Override

Closes #936

Closes #940

Closes #949

First iteration of procedures modal with block workspace