Use aria disclosure pattern in vertical toolbar submenu toggle buttons.

Closes #3093378

core/modules/toolbar/css/toolbar.icons.theme.css

@@ -275,7 +275,6 @@
 
 .toolbar .toolbar-icon.toolbar-handle {
   width: 4em;
-  text-indent: -9999px;
 }
 
 .toolbar .toolbar-icon.toolbar-handle::before {