MDL-62680 output: Only hide icons with no label

For accessibility we don't want to read an icon with a label immediately next to the label,
but in this case it's clearer for the icon to have no alt text / title for both
screen readers and non-screen readers. Worse is not reading important information just
because it's displayed as an icon.

lib/outputcomponents.php

@@ -694,6 +694,11 @@ class pix_icon implements renderable, templatable {
             // and some browsers might overwrite it with an empty title.
             unset($this->attributes['title']);
         }
+
+        // Hide icons from screen readers that have no alt.
+        if (empty($this->attributes['alt'])) {
+            $this->attributes['aria-hidden'] = 'true';
+        }
     }
 
     /**