Force use and update the job name with the loaded project file

CURA-4348

- If no project is loaded beforehand and then a model file is loaded,
  the job name should be determined with the current machine name and
  the first loaded model name.
- If a project is loaded, the job name should be the same as the project
  name, and it should not change until another project is loaded.

cura/CuraApplication.py

@@ -125,6 +125,8 @@ class CuraApplication(QtApplication):
     #        Cura will always show the Add Machine Dialog upon start.
     stacksValidationFinished = pyqtSignal()  # Emitted whenever a validation is finished
 
+    projectFileLoaded = pyqtSignal(str)  # Emitted whenever a project file is loaded
+
     def __init__(self):
         # this list of dir names will be used by UM to detect an old cura directory
         for dir_name in ["extruders", "machine_instances", "materials", "plugins", "quality", "user", "variants"]: