Update management of body selection.

CMakeLists.txt

include/gepetto/gui/bodytreewidget.hh

include/gepetto/gui/fwd.hh

include/gepetto/gui/mainwindow.hh

include/gepetto/gui/osgwidget.hh

include/gepetto/gui/pick-handler.hh

include/gepetto/gui/qt-osg-keyboard.hh

include/gepetto/gui/selection-event.hh

include/gepetto/gui/selection-handler.hh

src/gui/CMakeLists.txt

src/gui/bodytreewidget.cc

src/gui/mainwindow.cc

src/gui/osgwidget.cc

src/gui/pick-handler.cc

src/gui/pythonwidget.cc

src/gui/selection-event.cc

src/gui/selection-handler.cc