Add the files transfered Widget

For now it's quite simple, and it's lacking a bit of functionality: the ToxFile struct doesn't have any path information, so clicking on the files fails to open them (excepting the hack for downloads, which would be broken if the client allowed custom download locations).

So, if anyone knows more about file transfers and can somehow locate the full path for the widget, that would be great.

Also Flynn (or whoever does these things), a better checkmark icon wouldn't be bad (and I'm not even sure it works currently).