The new logic will select GTK+3 by default if WITH_GTK is selected. If
the GTK+3 libraries are not found, then GTK+2 libraries will be selected
if found.
This can be overridden by using WITH_GTK_2_X to force selection of GTK+2
(if found).

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

Rebase to lates upstream

Conflicts:
	CMakeLists.txt