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/cmakeprojectmanager/cmakebuildsystem.h

@@ -55,6 +55,13 @@ protected:
     bool validateParsingContext(const ParsingContext &ctx) final;
     void parseProject(ParsingContext &&ctx) final;
 
+    bool supportsAction(ProjectExplorer::Node *context,
+                        ProjectExplorer::ProjectAction action,
+                        const ProjectExplorer::Node *node) const override;
+
+    bool addFiles(ProjectExplorer::Node *context,
+                  const QStringList &filePaths, QStringList *) final;
+
 private:
     // Treescanner states:
     void handleTreeScanningFinished();