changing from slow to instant start by adding torrents in the background

because it allow using the program instantly (f.e. downloading new torrents) after its started

## adding my torrents hang the gui for 1 h without this patch

i have 7000 torrents in my list (6000 paused) because i use my torrent list as my library and backup (if i lose or delete a file i can download it again by starting the torrent)

adding my 7000 torrents takes 1 h during which the qbittorrent.exe gui thread is hanged so I have to wait 1 h after i start qbittorrent before i can download new files


## gui should never hang

a gui should never **hang or semi-hang** because

* it prevent the user from using the gui for other tasks while the slow task complete

a gui should never **hang** because

* the user don't know if the user should wait for the program to respond or restart it