These changes introduce Gtk3 support into the highgui module.

A new option WITH_GTK3 has been added to the cmake configure system to enable compilation under Gtk version 3.The flag HAVE_GTK3 is also introduced to select the new Gtk3 code in the source files. (Gtk2 is disbled when Gtk3 is selected).

window_gtk.cpp has been modified to remove obsolete (and deleted from libgtk 3) and introduce new Gtk3 code in its place when compiled for Gtk3.

To compile for Gtk2, disable WITH_GTK3 in cmake. To build for Gtk3 both WITH_GTK and WITH_GTK3 must be selected.
Disable OpenGL when Gtk3 is selected

The OpenGL library is not supported on Gtk3 which uses the cairo
library for rendering. Cairo, in turn uses hardware acceleration when
supported by the underlying system. Some improvement to performance
may be gained by useing the cairo-gl and cairo-glx libraries included
on modern Gtk3 systems.

See the cairo documentation for further information. The following
link may also be of interest.

http://lists.cairographics.org/archives/cairo/2012-October/023609.html
Code cleanup

Remove redundent #if defs from code
Rework gtk3 window resize and draw cleanup

The gtk3 resize functions fail to shrink the window after it has been
resized. This edit changes this and allows the window to be shrunk
back to its original size.

This edit also cleans up the draw/expose functions.
mprove Gtk2/3 options in cmake

Update to cmake files for to include minimum versions, and tidy up gtk operation.

Files updated:
CMakeLists.txt:
  WITH_GTK now uses Gtk3 by default. If not found then Gtk2 is used.
  WITH_GTK_2_X forces Gtk2.x use

cmake/OpenCVFindLibsGUI.cmake
  Updated selection logic to implement methodology described above.
  Implemented warning if Gtk3 not found (and not overridden)
  Implemented error if Gtk does not meet minimum required version

cmake/OpenCVMinDepVersions.cmake
  Added minimum Gtk version of 2.18.0
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
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
Improve GTK+ library selection logic.

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).
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master' into gtk3

Conflicts:
	CMakeLists.txt
Fix bug in GTK+3 logic introduced by previous merge

During merging of conflicting versions of this file, I erroneously
deleted several lines in the GUI reporting section. This is repaired
in this commit.
Change quotes around GTK headers for angle brackets

The linux buildbots have started to fail compilation due to not
finding the gtk headers. The quotes have been changed to angle brackets
to indicate to the compiler that these are system includes.
Correct error with GTK3 not found selection

When GTK3 is not found, HAVE_GTK was being set to TRUE. This edit
ensures that HAVE_GTK is only set if GTK3 or GTK2 (meeting minimum
version requirements) is present.
Selection logic for printing 'GTK: No' when the libraries are not found
has also been removed so the message is printed when GTK is not found or
selected in common with other libraries.

Changes committed:
	modified:   CMakeLists.txt
	modified:   cmake/OpenCVFindLibsGUI.cmake
Merge remote-tracking branch 'upstream/master'

Conflicts:
	CMakeLists.txt
Merge remote-tracking branch 'upstream/master' into gtk3

Conflicts:
	CMakeLists.txt
Fix logic error in OpenCVFindLibsGUI.cmake

When with_gtk is selected but GTK3 is not present the current logic
fails to check for GTK2. This edit corrects this.
Correction to enable compilation on platform with only GTK2 libs

	modified:   CMakeLists.txt
	modified:   cmake/OpenCVFindLibsGUI.cmake
Merge remote-tracking branch 'upstream/master'
Rebase to lates upstream