feat: Implement a 'Exit 2D MPR' button in the toolbar

* feat: Implement a 'Exit 2D MPR' button in the toolbar to act as a toggle when '2D MPR' button is clicked

* Move logic to exit MPR and toggling into ConnectedPluginSwitch and PluginSwitch. Also, discard the ConnectedExitPluginSwitch and ExitPluginSwitch components.

* Update ToolbarRow.js

platform/viewer/src/connectedComponents/PluginSwitch.js

@@ -6,14 +6,43 @@ import './PluginSwitch.css';
 class PluginSwitch extends Component {
   static propTypes = {
     mpr: PropTypes.func,
+    exitMpr: PropTypes.func
   };
 
   static defaultProps = {};
+  constructor(props) {
+    super(props);
+    this.state = {
+      isPlugSwitchOn: false,
+      label: "2D MPR",
+      icon: "cube"
+    };
+  }
+
+  handleClick = () => {
+    if (this.state.isPlugSwitchOn) {
+      this.setState({
+        isPlugSwitchOn: false,
+        label: "2D MPR",
+        icon: "cube"
+      });
+      this.props.exitMpr();
+    } else {
+      this.setState({
+        isPlugSwitchOn: true,
+        label: "Exit 2D MPR",
+        icon: "times"
+      });
+      this.props.mpr();
+    }
+  };
 
   render() {
+    const { label, icon } = this.state;
+
     return (
       <div className="PluginSwitch">
-        <ToolbarButton label="2D MPR" icon="cube" onClick={this.props.mpr} />
+        <ToolbarButton label={label} icon={icon} onClick={this.handleClick} />
       </div>
     );
   }