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/nim/project/nimprojectnode.h

@@ -27,26 +27,12 @@
 
 #include <projectexplorer/projectnodes.h>
 
-namespace Utils { class FilePath; }
-
 namespace Nim {
 
-class NimBuildSystem;
-
 class NimProjectNode : public ProjectExplorer::ProjectNode
 {
 public:
     NimProjectNode(const Utils::FilePath &projectFilePath);
-
-    bool supportsAction(ProjectExplorer::ProjectAction action, const Node *node) const override;
-    bool addFiles(const QStringList &filePaths, QStringList *) override;
-    ProjectExplorer::RemovedFilesFromProject removeFiles(const QStringList &filePaths,
-                                                         QStringList *) override;
-    bool deleteFiles(const QStringList &) override;
-    bool renameFile(const QString &filePath, const QString &newFilePath) override;
-
-private:
-    NimBuildSystem *buildSystem() const;
 };
 
 }