Merge pull request #2189 from ProjectMirador/thumbnail-nav-companion-area

__tests__/integration/mirador/thumbnail-navigation.test.js

__tests__/src/actions/window.test.js

__tests__/src/components/CompanionWindowFactory.test.js

__tests__/src/components/ThumbnailNavigation.test.js

__tests__/src/components/Window.test.js

__tests__/src/components/WindowThumbnailSettings.test.js

__tests__/src/lib/MiradorViewer.test.js

__tests__/src/reducers/windows.test.js

__tests__/src/selectors/index.test.js

src/components/CompanionArea.js

src/components/CompanionWindowFactory.js

src/components/ThumbnailNavigation.js

src/components/Window.js

src/components/WindowThumbnailSettings.js

src/config/settings.js

src/containers/ThumbnailNavigation.js

src/state/actions/window.js

src/state/reducers/windows.js

src/state/selectors/index.js