All the Qt5 setup looks fine. I think your problem is the add_dependencies line, since it doesn't seem to be properly directing CMake to build your messages before trying to build your node. I use gencpp instead of generate_messages_cpp when I do things like this, and it seems to work fine. Therefore, I would try this add_dependencies command instead:
add_dependencies(demointerface demointerface_gencpp)
On a side note, you can also use the ${PROJECT_NAME} macro to insert the name of the ROS package. This way, it's typo-proof, and will adapt if you ever change the name of the package for some reason. Using this, the above could be written as: add_dependencies(${PROJECT_NAME} \${PROJECT_NAME}_gencpp)