fix is-withicon when used with is-grouped on controls

Author: mna

bulma/elements/controls.sass

@@ -183,14 +183,14 @@
   &:not(:last-child)
     margin-bottom: 10px
   &.is-withicon
-    .fa
+    .input + .fa
       +fa(14px, 20px)
       color: $text-light
       left: 6px
       pointer-events: none
       position: absolute
       top: 6px
-      z-index: 1
+      z-index: 4
     .input
       padding-left: 32px
       &:focus + .fa