CPlusPlus: CppEditor: refactor overview model

Introduce abstract model to be able to use clang
based version of it in follow up patches.
Fix warnings and modernize source code a little.
Move OverviewModel to CppTools.

Change-Id: Idcc9bf03cad047026a456bd01063597a1eb95147
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/libs/cplusplus/cplusplus.qbs

@@ -113,7 +113,6 @@ Project {
                 "MatchingText.cpp", "MatchingText.h",
                 "NamePrettyPrinter.cpp", "NamePrettyPrinter.h",
                 "Overview.cpp", "Overview.h",
-                "OverviewModel.cpp", "OverviewModel.h",
                 "PPToken.cpp", "PPToken.h",
                 "PreprocessorClient.cpp", "PreprocessorClient.h",
                 "PreprocessorEnvironment.cpp", "PreprocessorEnvironment.h",