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.qbs

@@ -71,6 +71,7 @@ QtcPlugin {
         "doxygengenerator.cpp", "doxygengenerator.h",
         "functionutils.cpp", "functionutils.h",
         "includeutils.cpp", "includeutils.h",
+        "indexitem.cpp", "indexitem.h",
         "insertionpointlocator.cpp", "insertionpointlocator.h",
         "searchsymbols.cpp", "searchsymbols.h",
         "stringtable.cpp", "stringtable.h",