Doc: reorganize items in the 2.7 change log

Move two debugging items from "C++ Support" to
"Debugging".
Create subsections for "Code completion" and "C++11
support" in "C++ Support".
Try to list related items together.

Change-Id: Id3cb1ac54ac1226adbc7a5d0413f6a5142f3365d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>