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/qmakeprojectmanager/qmakeproject.cpp

@@ -145,6 +145,7 @@ QmakeProject::QmakeProject(const FilePath &fileName) :
             this, &QmakeProject::buildFinished);
 
     setPreferredKitPredicate([this](const Kit *kit) -> bool { return matchesKit(kit); });
+    setBuildSystem(std::make_unique<QmakeBuildSystem>(this));
 }
 
 QmakeProject::~QmakeProject()