stlgeometry is a private member of VisualSceneSTLGeometry/Meshing, and
can't point to any valid geometry when the constructor runs, so remove
any dereferences there.
Remove the misleading extern declaration of netgen::stlgeometry.
move fieldlines code to meshing dir
move visualization function pointers to meshing directory
DLL_HEADER -> NGGUI_API in visualization lib
move soldata.hpp to meshing
update occ, no freetype necessary anymore