Python: improve initializing python run configuration

Sets main.py as the default script. If we cannot find a main.py we
create a runconfig for every py file in the project.

Change-Id: I93f4c9a5c9e2825a592eee3707439e69a8aea566
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/python/pythonproject.cpp

@@ -261,6 +261,7 @@ void PythonBuildSystem::triggerParsing()
             bti.buildKey = f;
             bti.targetFilePath = filePath;
             bti.projectFilePath = projectFilePath();
+            bti.isQtcRunnable = filePath.fileName() == "main.py";
             appTargets.append(bti);
         }
     }