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/qmlprojectmanager/qmlproject.cpp

@@ -67,7 +67,7 @@ QmlProject::QmlProject(const Utils::FilePath &fileName)
     setDisplayName(fileName.toFileInfo().completeBaseName());
 
     setNeedsBuildConfigurations(false);
-    setBuildSystem(std::make_unique<Internal::QmlBuildSystem>(this));
+    setBuildSystemCreator([](Project *p) { return new Internal::QmlBuildSystem(p); });
 
     connect(this, &QmlProject::projectFileIsDirty, this, &QmlProject::refreshProjectFile);
 }