Issue #3444071: move term_merge related function to submodule

This reverts commit 33f429d6, f91325a7 and 4c362a41.

Closes #3444071

js/tree.js

@@ -88,25 +88,26 @@
       },
       source: source,
       select: function (event, data) {
-        // We update the form inputs on every checkbox state change as
-        // ajax events might require the latest state.
+        // We update the form inputs on every checkbox state change as ajax
+        // events might require the latest state.
         data.tree.generateFormElements(name + '[]');
         // If no item is selected then disable delete button.
         if (data.tree.getSelectedNodes().length < 1) {
           document.getElementById("edit-delete").disabled = true;
-          document.getElementById("edit-merge").disabled = true;
         } else {
-          let $deleteButton = document.getElementById("edit-delete"),
-              $mergeButton = document.getElementById("edit-merge");
+          let $deleteButton = document.getElementById("edit-delete");
           $deleteButton.disabled = false;
-          $mergeButton.disabled = false;
           if ($deleteButton.classList.contains('is-disabled')) {
             $deleteButton.classList.remove('is-disabled');
           }
-          if ($mergeButton.classList.contains('is-disabled')) {
-            $mergeButton.classList.remove('is-disabled');
-          }
         }
+
+        // Create custom event for tree selection so other modules are able to
+        // react on selection changes.
+        let treeSelectEvent = new CustomEvent('taxonomy_manager-tree-select', {
+          detail: data
+        });
+        document.dispatchEvent(treeSelectEvent);
       },
       focus: function (event, data) {
         new Drupal.TaxonomyManagerTermData(data.node.key, data.tree);