Unexports CppModelManager, CppSourceProcessor and CppToolsPlugin.

Now only some constructor signatures mention "Internal::" in the
exported symbols:

% nm --extern-only --demangle ./lib/qtcreator/plugins/libCppTools.so | grep "Internal::"
CppTools::CppClassesFilter::CppClassesFilter(CppTools::Internal::CppLocatorData*)
CppTools::CppClassesFilter::CppClassesFilter(CppTools::Internal::CppLocatorData*)
CppTools::CppEditorSupport::CppEditorSupport(CppTools::Internal::CppModelManager*, TextEditor::BaseTextEditor*)
CppTools::CppEditorSupport::CppEditorSupport(CppTools::Internal::CppModelManager*, TextEditor::BaseTextEditor*)

Change-Id: I167c21a6dc03cf02230c95fde66cf404e40df36f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>