MC-3477: Polish prototype and apply rearranging in preview

- Update draggable handle style to use variables

Author: Hwashiang Yu

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

@@ -38,9 +38,9 @@
                     cursor: -moz-grab;
                     cursor: -webkit-grab;
                     display: inline-block;
-                    font-size: 16px;
-                    left: -16px;
-                    line-height: 2.2rem;
+                    font-size: @pagebuilder-button__font-size;
+                    left: -(@pagebuilder-button__font-size);
+                    line-height: @pagebuilder-button__line-height;
                     position: relative;
                     width: 0;