If you have nix installed compile and run with:
$ nix build
$ ./result/bin/demo
Explaination: we load the .qml file with qrc:///main.qml: it uses for that the file qml.qrc (the name is not important) that lists the resource files. This 'qml.qrc file is then given to resources in meson.build in order to compile the resources and include them in the final binary.