Changes can be documented in two ways now: Either by directly editing the
CHANGES file like it was done until now or by storing each entry for the
CHANGES file correctly formated in a separate file in the changes-entries
directory.

The benefit of the single file per change approach is that it eases backporting
the CHANGES entry to a stable branch as it avoids the frequent merge conflicts
as changes are merged in different orders or not at all in the stable branch.

In order to keep the current CHANGES file for the users as is there is a new
make target called 'update-changes'. It merges all change files in the
changes-entries directory to the top of the CHANGES file and removes them
afterwards.

This make target can be seen in a similar way as the scripts to update the
documentation files from its xml sources. It can be executed immediately
after the new file in the changes-entries directory has been created / merged
and committed or it can be executed later. It should be executed at least before
a release gets tagged.