basic plugin tests (delete, replace, wrap, add)

__tests__/integration/mirador/plugins/add.html

__tests__/integration/mirador/plugins/add.test.js

__tests__/integration/mirador/plugins/delete.html

__tests__/integration/mirador/plugins/delete.test.js

__tests__/integration/mirador/plugins/replace.html

__tests__/integration/mirador/plugins/replace.test.js

__tests__/integration/mirador/plugins/wrap.html

__tests__/integration/mirador/plugins/wrap.test.js

src/components/WorkspaceControlPanelButtons.js