add ItemManager::findMovie

This commit also fix several errors :
  - treeRoot is the default parent for Item creation in ItemManager.
  - Folder's subItems lists are well updated after child additions / deletions.
  - Folder::(remove/delete)SubItem do its action (respectively remove the item from the child list and delete the item) before list modification
    - This bug created several problems like removing the given Item but setting the parent of the next Item in the list to nullptr.