[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.
> 
> 
> BTW did get to it? Pavel


Pavel, I didn't forget about this and have some progress in this 
direction. It is in my lyx-todo list just after completing with 2 
branches in features repository. Since it was announced that only 
bug-fixing patches are allowed in master branch, I paused all 
development/refactoring/cleanup activities till the release of version 
2.4.0.


Yuriy