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
Updating STAR detector and FREAK descriptor to work with large and/or 16-bit images
Don't automatically convert to grayscale (keep previous behavior.)
Merge branch 'master' of https://github.com/Itseez/opencv

Conflicts:
	modules/imgproc/src/sumpixels.cpp
Pick centers in KMeans++ with a probability proportional to their distance^2, instead of simple distance, to previous centers
Apply to KMeansIndex KMeanspp the same modification as in HierarchicalClusteringIndex
As some processed distances are already ^2, use template to select whether or not we have to ^2 in KMeanspp
Move templates in dist.h in order to share them between KMeansIndex and HierarchicalClusteringIndex classes.
When a cluster is empty for KMeans, it's better to give it the point from another cluster j that is the furthest one from center j.
Add a new method for initializing KMeans centers that leads to better clusters and thus better retrieval when final centers have to be existing keypoints instead of clusters barycenters.
converted necessary files for python3 compatibility using 2to3
fixed relative imports in Jinja for python3
explicit string encoding when writing to file in python3
added epsilon value to weights in the MergeMertens in order to avoid zero weights for pixels from uniformly filled areas of image
added test for correct handling of uniforma areas in the MergeMertens
New KNN code -should compile now