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-lib.pri

@@ -17,15 +17,13 @@ HEADERS += \
     $$PWD/Icons.h \
     $$PWD/ExpressionUnderCursor.h \
     $$PWD/BackwardsScanner.h \
-    $$PWD/MatchingText.h \
-    $$PWD/OverviewModel.h
+    $$PWD/MatchingText.h
 
 SOURCES += \
     $$PWD/Icons.cpp \
     $$PWD/ExpressionUnderCursor.cpp \
     $$PWD/BackwardsScanner.cpp \
-    $$PWD/MatchingText.cpp \
-    $$PWD/OverviewModel.cpp
+    $$PWD/MatchingText.cpp
 }
 
 HEADERS += \