Split STAGE_SIZE enum into two & clarify purposes

src/components/gui/gui.jsx

src/components/sprite-info/sprite-info.jsx

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

src/components/stage-header/stage-header.jsx

src/components/stage-wrapper/stage-wrapper.jsx

src/components/stage/stage.jsx

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

src/containers/blocks.jsx

src/containers/stage-header.jsx

src/containers/stage-wrapper.jsx

src/containers/stage.jsx

src/lib/layout-constants.js

src/lib/screen-utils.js

src/reducers/stage-size.js