C++: introduce the unique ProjectPart::id()

And change the code-model inspector dialog to use them in the
ProjectParts tab to sync the master/details view, and in the
CppPreProcessorDialog to save/reload the selected part.

Change-Id: I7cc8ac1c21d31122f8650985d0b85177ebabdfb8
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cppprojects.cpp

@@ -113,6 +113,11 @@ ProjectPart::Ptr ProjectPart::copy() const
     return Ptr(new ProjectPart(*this));
 }
 
+QString ProjectPart::id() const
+{
+    return QDir::fromNativeSeparators(projectFile) + QLatin1Char(' ') + displayName;
+}
+
 QByteArray ProjectPart::readProjectConfigFile(const ProjectPart::Ptr &part)
 {
     QByteArray result;