Add Exit icon.

This adds an "Exit" icon, which is used to exit fullscreen mode in Gutenberg.

See discussion in WordPress/gutenberg#9838 (comment).

react/example.jsx

@@ -157,6 +157,7 @@ module.exports = React.createClass( {
 				<Dashicon icon="gutenberg/embed-post" size={ 40 } onClick={ this.handleClick.bind( this, 'gutenberg/embed-post' ) } />
 				<Dashicon icon="gutenberg/embed-video" size={ 40 } onClick={ this.handleClick.bind( this, 'gutenberg/embed-video' ) } />
 				<Dashicon icon="excerpt-view" size={ 40 } onClick={ this.handleClick.bind( this, 'excerpt-view' ) } />
+				<Dashicon icon="gutenberg/exit" size={ 40 } onClick={ this.handleClick.bind( this, 'gutenberg/exit' ) } />
 				<Dashicon icon="external" size={ 40 } onClick={ this.handleClick.bind( this, 'external' ) } />
 				<Dashicon icon="facebook-alt" size={ 40 } onClick={ this.handleClick.bind( this, 'facebook-alt' ) } />
 				<Dashicon icon="facebook" size={ 40 } onClick={ this.handleClick.bind( this, 'facebook' ) } />

react/index.jsx

@@ -437,6 +437,9 @@ export default class Dashicon extends Component {
 			case 'excerpt-view':
 				path = 'M19 18V2c0-.55-.45-1-1-1H2c-.55 0-1 .45-1 1v16c0 .55.45 1 1 1h16c.55 0 1-.45 1-1zM4 3c.55 0 1 .45 1 1s-.45 1-1 1-1-.45-1-1 .45-1 1-1zm13 0v6H6V3h11zM4 11c.55 0 1 .45 1 1s-.45 1-1 1-1-.45-1-1 .45-1 1-1zm13 0v6H6v-6h11z';
 				break;
+			case 'exit':
+				path = 'M13 3v2h2v10h-2v2h4V3h-4zm0 8V9H5.4l4.3-4.3-1.4-1.4L1.6 10l6.7 6.7 1.4-1.4L5.4 11H13z';
+				break;
 			case 'external':
 				path = 'M9 3h8v8l-2-1V6.92l-5.6 5.59-1.41-1.41L14.08 5H10zm3 12v-3l2-2v7H3V6h8L9 8H5v7h7z';
 				break;