Smb4K 4.0.0 will be Qt6 / KF6 only

This week I decided to make Smb4K 4.0 a Qt6 / KF6 only application, so that I do not have to maintain code for two major versions of the Qt library. However, I also decided to start working on the 3.2 branch again, after the remaining big things in the development branch are done (updating the handbook, thorough testing, etc.). So, I guess there will be at least a version 3.2.6 in the near future,