MC-3392: Hide Content block/ container from the stage

- Fix issue with tabs active state

Author: davemacaulay

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/_preview.less

@@ -166,6 +166,7 @@
         height: 100%;
         justify-content: center;
         left: 0;
+        line-height: 0;
         opacity: 0;
         position: absolute;
         text-shadow: 0 0 25px rgba(238, 238, 238, .7);

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

@@ -26,9 +26,12 @@
             content: '';
             height: 1px;
             left: -1px;
+            opacity: 1;
             position: absolute;
             top: -2px;
+            visibility: visible;
             width: calc(~'100% + 2px');
+            z-index: auto;
         }
 
         &.empty-container {