Improve GTK+ library selection logic.
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).

opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv