ProjectExplorer: Use a functor for BuildSystem creation

... instead of creating the BuildSystem direct. This will help the
shift of BuildSystem owner ship as a Project will have potentially
multiple BuildSystem instances (one per BuildConfiguration), but
still be responsible for creating them with the Targets.

Change-Id: I2dd71c7687ed41af9e42c874b3f932ce704e7ee3
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/qbsprojectmanager/qbsproject.cpp

@@ -135,7 +135,7 @@ QbsProject::QbsProject(const FilePath &fileName)
     setProjectLanguages(Context(ProjectExplorer::Constants::CXX_LANGUAGE_ID));
     setCanBuildProducts();
 
-    setBuildSystem(std::make_unique<QbsBuildSystem>(this));
+    setBuildSystemCreator([](Project *p) { return new QbsBuildSystem(p); });
 
     rebuildProjectTree();