Introduced a token cache for the C++ editor.

This should speed things up a bit, because before, the line was tokenized at
least 3 times.

src/libs/cplusplus/ExpressionUnderCursor.h

@@ -43,11 +43,12 @@ namespace CPlusPlus {
 
 class BackwardsScanner;
 class SimpleToken;
+class TokenCache;
 
 class CPLUSPLUS_EXPORT ExpressionUnderCursor
 {
 public:
-    ExpressionUnderCursor();
+    ExpressionUnderCursor(TokenCache *tokenCache);
     ~ExpressionUnderCursor();
 
     QString operator()(const QTextCursor &cursor);
@@ -59,6 +60,7 @@ private:
     bool isAccessToken(const SimpleToken &tk);
 
 private:
+    TokenCache *_tokenCache;
     bool _jumpedComma;
 };