Converting Task to a Tree structure (#11117)

* added support for tree view

* nestedset added to handle tree based structure

* treeview ui added

* removed is_group dependency

* added validation while editing a group-task

* codacy fix

* BOM like filter added

* Added ui-test for treeview-task
diff --git a/erpnext/tests/ui/tests.txt b/erpnext/tests/ui/tests.txt
index e7de604..24858f3 100644
--- a/erpnext/tests/ui/tests.txt
+++ b/erpnext/tests/ui/tests.txt
@@ -133,3 +133,4 @@
 erpnext/restaurant/doctype/restaurant_table/test_restaurant_table.js
 erpnext/restaurant/doctype/restaurant_menu/test_restaurant_menu.js
 erpnext/restaurant/doctype/restaurant_order_entry/restaurant_order_entry.js
+erpnext/projects/doctype/task/tests/test_task_tree.js