MAGETWO-92379: Option Menus Display Behind Button

- Fallback to default stacking order in Button Item

Author: tjwiebell

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/button-item/_default.less

@@ -10,6 +10,7 @@
 .pagebuilder-content-type {
     .pagebuilder-button-item {
         width: auto;
+        z-index: auto;
         [contenteditable=true] {
             display: inline-block;
             min-height: 18px;