basic per-window visibility (to be refined...)

Fltk/GUI.cpp

Fltk/Makefile

Fltk/extraDialogs.cpp

Fltk/openglWindow.cpp

Fltk/openglWindow.h

Fltk/visibilityWindow.cpp

Fltk/visibilityWindow.h

Graphics/drawContext.h

Graphics/drawGeom.cpp

Graphics/drawGraph2d.cpp

Graphics/drawMesh.cpp

Graphics/drawPost.cpp

Graphics/drawScales.cpp