Initial rough sprite/costume/backdrop libraries

package.json

src/components/costume-canvas.js

src/components/library-item.js

src/components/library.js

src/components/modal.js

src/components/sprite-selector.js

src/containers/backdrop-library.js

src/containers/costume-library.js

src/containers/gui.js

src/containers/sprite-library.js

src/containers/sprite-selector.js

src/lib/media-library.js