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

[Jean-Marc Lasgouttes]

Le 16/12/2020 à 20:37, Yuriy Skalko a écrit :
> Great! It is now only 1.5 MB!
> 
> Using "signals" conflicts with Qt keyword, so I've used "signal". Also 
> moved now unneeded for signals class Trackable to the only place where 
> it is used.

Since support/signals.h is pretty empty now, why not just use 
nod/signal.h and nod::signal? Then we can get rid of the header file.

Patches 2 and 4 are fine.

JMarc