Qbs, Qmake, Python, Generic: Introduce BuildSystem derived classes

... and move context menu action handling there.

This is a temporary measure to be able to move that functionality
alongside the actual BuildSystem to the BuildConfiguration.

There is a lot to be cleaned up left, to keep the patch small.

Change-Id: If4b0820a13b376fc97b70785052924972ce22705
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmlprojectmanager/qmlproject.cpp

@@ -67,6 +67,7 @@ QmlProject::QmlProject(const Utils::FilePath &fileName)
     setDisplayName(fileName.toFileInfo().completeBaseName());
 
     setNeedsBuildConfigurations(false);
+    setBuildSystem(std::make_unique<Internal::QmlBuildSystem>(this));
 
     connect(this, &QmlProject::projectFileIsDirty, this, &QmlProject::refreshProjectFile);
 }
@@ -398,5 +399,6 @@ void QmlProject::updateDeploymentData(ProjectExplorer::Target *target)
 
     target->setDeploymentData(deploymentData);
 }
+
 } // namespace QmlProjectManager