Update to use a MiradorIcon component rather than the text button

__tests__/miradorAnnotationPlugin.test.js

 import React from 'react';
 import { shallow } from 'enzyme';
-import Button from '@material-ui/core/Button';
+import { MiradorMenuButton } from 'mirador/dist/es/src/components/MiradorMenuButton';
 import miradorAnnotationPlugin from '../src/plugins/miradorAnnotationPlugin';
 import AnnotationCreation from '../src/AnnotationCreation';
 
@@ -20,14 +20,14 @@ describe('MiradorAnnotation', () => {
   let wrapper;
   it('renders a create new button', () => {
     wrapper = createWrapper();
-    expect(wrapper.find(Button).text()).toBe('Create New');
+    expect(wrapper.find(MiradorMenuButton).props()['aria-label']).toBe('Create new annotation');
   });
   it('opens a new companionWindow when clicked', () => {
     const mockAddCompanionWindow = jest.fn();
     wrapper = createWrapper({
       addCompanionWindow: mockAddCompanionWindow,
     });
-    wrapper.find(Button).simulate('click');
+    wrapper.find(MiradorMenuButton).simulate('click');
     expect(mockAddCompanionWindow).toHaveBeenCalledWith(
       'custom',
       {

src/plugins/miradorAnnotationPlugin.js

 import React, { Component } from 'react';
 import PropTypes from 'prop-types';
 import * as actions from 'mirador/dist/es/src/state/actions';
-import Button from '@material-ui/core/Button';
+import AddBoxIcon from '@material-ui/icons/AddBox';
+import { MiradorMenuButton } from 'mirador/dist/es/src/components/MiradorMenuButton';
 import { getVisibleCanvases } from 'mirador/dist/es/src/state/selectors/canvases';
 import AnnotationCreation from '../AnnotationCreation';
 
@@ -42,9 +43,12 @@ class MiradorAnnotation extends Component {
           {...targetProps} // eslint-disable-line react/jsx-props-no-spreading
         />
         <div>
-          <Button variant="contained" color="primary" size="small" onClick={this.openCreateAnnotationCompanionWindow}>
-            Create New
-          </Button>
+          <MiradorMenuButton
+            aria-label="Create new annotation"
+            onClick={this.openCreateAnnotationCompanionWindow}
+          >
+            <AddBoxIcon />
+          </MiradorMenuButton>
         </div>
       </div>
     );