Use GTK_VERSION_MAJOR to switch between  GTK+ version two and three
specific code. As a result of this, HAVE_GTK3 no longer needs to be
exposed.

The use of HAVE_GTK, and HAVE_ GTK3 have been changed to mirror the
method used by HAVE_QT and HAVE_QT5.
 On branch gtk3
 Changes to be committed:
        modified:   CMakeLists.txt
        modified:   cmake/OpenCVFindLibsGUI.cmake
        modified:   cmake/templates/cvconfig.h.in
        modified:   modules/highgui/src/window.cpp
        modified:   modules/highgui/src/window_gtk.cpp