improve compatibility for qt 6.

[xhawk18]

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders=linux,docs,Custom
build_image:Custom=qt:16.04
buildworker:Custom=linux-1

[vpisarev]

@xhawk18, can you please re-submit the pull request from another branch? no need to close this PR, just create a dedicated branch, derived from fresh 3.4, and put your changes there. Currently we have a merge conflict, and because PR is submitted from 3.4 branch, the patch is not tested automatically.

[asmorkalov]

@xhawk18 Friendly reminder.