Continue aborted downloads

If updates downloading is aborted, it must be continued (when it is possible) not from the beginning, but using already downloaded data.
There are different reasons of abort:

1. UGENE is closed/crashed while downloading updates.
   When UGENE is started again it has to continue downloading.
2. Internet connection is lost.
   Every time period (e.g. 1 minute) UGENE must check if the connection is established. When it is established, the downloading must be continued.