Have a dedicated thread to process new videos.

When I add a new playlist or channel to 4kdownloader, it immediately starts to download videos, using all available threads. Lets say I am using 5 threads. It will download the first 5 videos, then and only then after it finished one of those 5 videos, will it start to search for more videos. If 4kdownloader had a dedicated thread for processing videos, then 4kdownloader could process every video in a few minutes, getting them ready to download. I ask for this, because when I close 4kdownloader, if it did not grab every video, then when I reopen it, it forgets that it didn't finish processing the playlist or channel. Making this change would make sure all videos that exist at the time of adding the channel are downloaded. To clarify, this is only a problem when downloading videos that already exist after restarting 4kdownloader. If a new video was uploaded, 4kdownload will recognize it and start downloading it.