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