Add config option to show manifest list. Closes #2759

__tests__/integration/mirador/index.html

__tests__/src/actions/config.test.js

__tests__/src/actions/workspace.test.js

__tests__/src/reducers/workspace.test.js

src/config/settings.js

src/containers/WindowListButton.js

src/containers/WorkspaceAddButton.js

src/containers/WorkspaceArea.js

src/containers/WorkspaceMenu.js

src/state/actions/config.js

src/state/actions/workspace.js

src/state/reducers/workspace.js