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/qbsprojectmanager/qbsproject.cpp

@@ -135,6 +135,8 @@ QbsProject::QbsProject(const FilePath &fileName)
     setProjectLanguages(Context(ProjectExplorer::Constants::CXX_LANGUAGE_ID));
     setCanBuildProducts();
 
+    setBuildSystem(std::make_unique<QbsBuildSystem>(this));
+
     rebuildProjectTree();
 
     connect(this, &Project::activeTargetChanged, this, &QbsProject::changeActiveTarget);