Fix building and installing translations

This broke in [1]. While ecm_create_qm_loader can take a target, it just doesn't exist at the time of invocation. Use the opportunity and introduce a more target-centric approach to address this. Furthermore, after the move of ecm_install_po_files_as_qm() to src/CMakeLists.txt (to avoid duplicate targets), the path passed to it needs adjustment.

[1] 8b335d87