Accessibility fixes

- Aria landmarks
 - Title attributes to all icon only buttons
 - <Multibutton/> now internally a radio group
 - Replaced 1 'skip navigation link' with UI group links
 - Added map specific shortcuts to the shortcut menu
 - Hidden layer list actions from tab index

src/components/fields/_DeleteStopButton.jsx

@@ -14,7 +14,7 @@ export default class DeleteStopButton extends React.Component {
     return <Button
       className="maputnik-delete-stop"
       onClick={this.props.onClick}
-      title={"Remove zoom level stop."}
+      title={"Remove zoom level from stop"}
     >
       <MdDelete />
     </Button>