C++: Fix handling of Objective-C/C++

- Objective C/C++ was not enabled in highlighter.
- QMake project part for Objective C/C++ did not have ObjectiveC extension enabled.
- As languageFeatures.objCEnabled is a bitfield, it was actually always set to 0.
- Highlight ObjC class & protocol declarations.
- Highlight ObjC message passing.

Change-Id: I64d12c9509058d05f7adce94598cb7ce91727ac8
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -497,6 +497,7 @@ void QmakeProject::updateCppCodeModel()
                                            ProjectFile::CXXSource));
         pinfo.appendProjectPart(cppPart);
         objcppPart->displayName += QLatin1String(" (ObjC++)");
+        objcppPart->languageExtensions |= ProjectPart::ObjectiveCExtensions;
         if (!objcppPart->files.isEmpty()) {
             pinfo.appendProjectPart(objcppPart);
             cppPart->displayName += QLatin1String(" (C++)");