Move add sprite and backdrop to their correct containers

src/components/sprite-selector/icon--sprite.svg

src/components/sprite-selector/sprite-selector.css

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

src/components/stage-selector/icon--backdrop.svg

src/components/stage-selector/stage-selector.css

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

src/components/target-pane/icon--add.svg

src/components/target-pane/target-pane.css

src/components/target-pane/target-pane.jsx

src/containers/stage-selector.jsx

src/containers/target-pane.jsx