[TASK] Add border to inactive drop areas (#684)

In the case of nested CEs, the drop areas touch each other, making it more difficult for the editor to figure out where to drop the CE.
Adding a border to the inactive drop areas solves this problem.

Resolves: #683

Author: webian

Resources/Private/Sass/inline_editing.scss

@@ -54,6 +54,7 @@
 
 .dropzones-enabled .t3-frontend-editing__dropzone {
     height: 25px;
+    border: solid 1px white;
 
     &.active {
         border: dashed 1px $gray;