C++: update project info once after finishing adding all project parts.

So that recalculating the information is not O(n^2) but linear.

Change-Id: I69903e0b5ad321d071804d782ad634a3f300e71a
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cppprojects.h

@@ -146,6 +146,7 @@ public:
     const QList<ProjectPart::Ptr> projectParts() const;
 
     void appendProjectPart(const ProjectPart::Ptr &part);
+    void finish();
     void clearProjectParts();
 
     const ProjectPart::HeaderPaths headerPaths() const;