[DF-4399] Implemented first draft notification mechanism for tasks status changes.
[DF-4399]
DownloadTaskDescription is class now.
Status reporting refactor.
Tasks should be placed inside heap and cleared by DownloadManager.
[DF-4399] Added done tasks queue. Search and clear API implemented.
[DF-4399] Code style corrections.
[DF-4399] Some API correction.