Merge pull request #1378 from paulkaplan/update-asset-icons

src/components/asset-button/asset-button.css

src/components/asset-button/asset-button.jsx

src/components/asset-panel/icon--add-backdrop-lib.svg

src/components/asset-panel/icon--add-blank-costume.svg

src/components/asset-panel/icon--add-costume-lib.svg

src/components/asset-panel/icon--add-sound-lib.svg

src/components/asset-panel/icon--add-sound-record.svg

src/components/asset-panel/selector.css

src/components/asset-panel/selector.jsx

src/components/gui/gui.css

src/components/gui/gui.jsx

src/components/gui/icon--extensions.svg

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/containers/costume-tab.jsx

src/containers/sound-tab.jsx