C++: Introduce an ordering of completion items.

For example locals and function argument names appear early whereas
macros and keywords are way down. That should move helpful
items to the top of the list.

Change-Id: Ibcfe973ebbac2e3bce9cafbe14dc4c71430ddca7
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cppcompletionassist.h

@@ -120,7 +120,8 @@ private:
                            const QIcon &icon = QIcon(),
                            int order = 0,
                            const QVariant &data = QVariant());
-    void addCompletionItem(CPlusPlus::Symbol *symbol);
+    void addCompletionItem(CPlusPlus::Symbol *symbol,
+                           int order = 0);
     void addSnippets();
     void addKeywords();
     void addMacros(const QString &fileName, const CPlusPlus::Snapshot &snapshot);