[KPhotoAlbum] restructuring video thumbnail generation with ffmpeg

[Tobias Leupold]

> PS:  I found two conventions for writing the {}-clauses:
> 
> a) (my preference)
> 
>      if (...) {
> 
>          do something
> 
>      } else {
> 
>          whatever
> 
>      }
> 
> b)
> 
>      if (...)
> 
>      {
> 
>          do something
> 
>      }
> 
> Which one is the preferred one in KPA?

I think we try to stick to the kdelibs coding style:
https://community.kde.org/Policies/Kdelibs_Coding_Style