merge-rests engraver doesn't merge dots

[Urs Liska]

Hello all,

On https://github.com/openlilylib/snippets/issues/123 it was reported
that the engraver merging rests
(https://github.com/openlilylib/snippets/tree/master/editorial-tools/merge-rests-engraver)
fails to merge dots of dotted rests. This can be seen here:
http://lilybin.com/r1jcpm/3

I think this should be quite easy to fix and update, but I don't know
how to do it in that engraver. So any solution would be welcome. If
you're an openLilyLib user/contributor you may do this directly as a
pull request.

Thanks
Urs