Code model: Fix Windows issues (duplicate matches).

Ensure the code model receives file names with clean paths ('/')
and fix paths in the relevant places of the code model. Pass on clean
paths from Cpp reference find and display them correctly in the search
window tooltip.

Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/libs/cplusplus/CppDocument.h

@@ -341,6 +341,7 @@ public:
     QStringList dependsOn(const QString &fileName) const;
 
     void insert(Document::Ptr doc);
+    Document::Ptr value(const QString &fileName) const;
 
     using _Base::insert;