Merge pull request #2323 from paulkaplan/reorderable-sprites

src/components/asset-panel/selector.css

src/components/asset-panel/selector.jsx

src/components/asset-panel/sortable-asset.jsx

src/components/drag-layer/drag-layer.css

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

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

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

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

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

src/containers/costume-tab.jsx

src/containers/paint-editor-wrapper.jsx

src/containers/sound-tab.jsx

src/containers/sprite-selector-item.jsx

src/containers/target-pane.jsx

src/lib/drag-constants.js

src/lib/drag-utils.js

src/lib/sortable-hoc.jsx

test/unit/util/drag-utils.test.js