CppTools: Remove dependency of RawProjectPart to ProjectFile

The feature to category files to ProjectFiles was used by the qmake
project manager to specify if the file is "active", and by the Qbs
project manager to avoid unnecessary MIME type checking.

Make these two different use-cases explicit in the API.

Change-Id: Ia5a7da37f100149366fc75060fe04687e15f2bd3
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cppprojectfile.h

@@ -50,6 +50,7 @@ public:
         OpenCLSource,
     };
 
+    static Kind classifyByMimeType(const QString &mt);
     static Kind classify(const QString &filePath);
 
     static bool isSource(Kind kind);