MAINT-5232: Extract LLInitClass, LLDestroyClass from llui/llui.h

to a new llcommon/llinitdestroyclass.h.
This mechanism is so general -- but has so many related moving parts -- that
(a) it deserves to be in a header file all its own, instead of conflated with
llui.h, and (b) it should be in llcommon where anyone can use it. It has no
dependencies whatsoever on llui or anything viewer-specific.
In this very changeset we changed one #include "llui.h" whose comment admits
that it was only dragged in for LLDestroyClass.