Fixed disappeared folders and tasks in rev. 214 / #3263

It was bug in implementation from rev. 201.

When adding new task to existing one (not folder) as subtask, then after that update folder and that subtask disappeared.

Effect: to see that folder and subtask, you have to view disappeared folder parent, etc. - weird behaviour.

-> Tom I think list vie should rely only on category filed, not partially on is_folder.

Reason: after task update is_folder was 0 and category = TCATEGORY_FOLDER. It should be is_folder = 1.