Adjust language in selectors and libraries. Resolves GH-1759

src/components/sprite-selector/sprite-selector.jsx

src/components/stage-selector/stage-selector.jsx

src/containers/backdrop-library.jsx

src/containers/costume-tab.jsx

src/containers/extension-library.jsx

src/containers/sound-library.jsx

src/containers/sound-tab.jsx

src/containers/sprite-library.jsx

test/integration/costumes.test.js

test/integration/sounds.test.js