C++: Rename ModelItemInfo to IndexInfo.

And move it into its own header and source files.

Change-Id: I37401badd819e028e1d767425759dc0ff27afe31
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cpptools.pro

@@ -50,6 +50,7 @@ HEADERS += \
     doxygengenerator.h \
     functionutils.h \
     includeutils.h \
+    indexitem.h \
     insertionpointlocator.h \
     searchsymbols.h \
     stringtable.h \
@@ -103,6 +104,7 @@ SOURCES += \
     doxygengenerator.cpp \
     functionutils.cpp \
     includeutils.cpp \
+    indexitem.cpp \
     insertionpointlocator.cpp \
     searchsymbols.cpp \
     stringtable.cpp \