fix #25497: merge matching non-auto unison note heads

Based on discussion from French forum, it was agreed there was a need to be able to merge unison note heads even in cases where one has dots and the other does not, or where one or both notes are stemless (including whole notes).  While the default remains to not merge in these cases, explicitly setting the note head type (as opposed to leaving it "Auto") will now have the effect of allowing merging.