C++: Fix/tune semantic highlighter result chunk size.

The fix: when finished with a FunctionDefinition, only flush when the
number of usages reaches the chunk size. This should prevent a lot of
chunks with a low number of usages for files with short methods.

The tuning: for files larger than 10000 lines, use a larger chunk size
to prevent the UI thread from having to re-layout/re-paint too often.

Change-Id: I419174d306b8380c6fa8402825767e26c73f62ec
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cppchecksymbols.h

@@ -175,6 +175,7 @@ private:
     QSet<QByteArray> _potentialStatics;
     QList<AST *> _astStack;
     QVector<Use> _usages;
+    int _chunkSize;
     unsigned _lineOfLastUsage;
     QList<Use> _macroUses;
 };