Fix Qt 5.15 deprecation warnings
Hi @gladk! In 0eda9996 you fixed warnings from CMake, but there were also warnings from the compiler.
I fixed them here by actually changing the code. If you don't want to carry a patch, you can just disable the warnings, e.g. by using QT_DISABLE_DEPRECATED_BEFORE macro.
Additionally, I fixed Lintian errors about broken references to make the pipeline pass.