Add modebar tabindex 0

Author: davidangarita1

src/components/modebar/modebar.js

@@ -44,6 +44,7 @@ proto.update = function(graphInfo, buttons) {
 
     this.element.setAttribute('id', modeBarId);
     this.element.setAttribute('role', 'toolbar');
+    this.element.setAttribute('tabindex', '0')
     this._uid = modeBarId;
     this.element.className = 'modebar modebar--custom';
     if(context.displayModeBar === 'hover') this.element.className += ' modebar--hover ease-bg';