chore(bpmn-rules): add function to check for same parent

lib/features/modeling/rules/BpmnRules.js

@@ -378,10 +378,7 @@ function canAttach(elements, target, source, position) {
 function canMove(elements, target) {
 
   // only move if they have the same parent
-  var sameParent = size(groupBy(elements, function(s) { return s.parent && s.parent.id; })) === 1;
-
-  // Remove attachers that already have their host in the lis
-  if (!sameParent) {
+  if (!haveSameParent(elements)) {
     return false;
   }
 
@@ -501,3 +498,13 @@ function canConnectBoundaryEvent(source, target) {
 
   return false;
 }
+
+/**
+ * Returns true if all elements have the same parent
+ *
+ * @param  {Array<djs.model.Base>} elements
+ * @return {Boolean}
+ */
+function haveSameParent(elements) {
+  return size(groupBy(elements, function(e) { return e.parent && e.parent.id; })) === 1;
+}