better deletion of widgets after feedback from Erco;

Common/Gmsh.cpp

Common/OS.cpp

Common/OpenFile.cpp

Fltk/FlGui.cpp

Fltk/FlGui.h

Fltk/fieldWindow.cpp

Fltk/graphicWindow.cpp

Fltk/onelabGroup.cpp

Fltk/pluginWindow.cpp

Fltk/statisticsWindow.cpp

Fltk/viewButton.cpp

Geo/Chain.cpp