CMake: Implement CMakeTargetNode::buildKey()

This always sets a node id, that is reasonably unique. So use it.

Change-Id: I9b1cb978f0d6edffabee4eafa5ec078a6a6827a6
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakeprojectnodes.h

@@ -72,6 +72,7 @@ public:
 
     bool showInSimpleTree() const final;
     QString tooltip() const final;
+    QString buildKey() const final;
 
     bool supportsAction(ProjectExplorer::ProjectAction action, const Node *node) const override;
     bool addFiles(const QStringList &filePaths, QStringList *notAdded) override;