[LyX/master] Reduce the amount of needed boost headers

[Yuriy Skalko]

> By the way, when you are already digging into the internal workings of
> preview loader, would it be hard to allow mutliple threads loading
> graphics images in backgrounds instead of single one?
> Documents with lot of figures take a long time to load fully and most
> of nowadays machines have multiples cores, to do that job in parallel.
> 
> Pavel

Yes, this should be really useful. I'll check it.

Yuriy