Changing native dialogs

This is a fix for the issue #19931.
As native dialogs cannot be displayed on unix due to qtfiledialog api limitations,
the option of setting 'use native dialogs' has been removed from preferences dialog.
Now, on unix qt file dialog will be used and on windows and mac, their native file dialogs will be used.

I haven't added any code, but still if there is any mistake, please let me know