Merge topic 'cmake-gui-qt5-only'

4037beb747 QtDialog: remove Qt4 definition
ce9dbceb42 QtDialog: remove Qt4-only code
8ea50749da QtDialog: use Qt5's imported targets
4ccc9921be QtDialog: use qt5 functions for special Qt sources
e4d6015460 QtDialog: remove Qt4 support

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !4836