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