Add ability to merge unison noteheads even for differing dots and for whole notes

Windows 7, GIT commit: e89b053

From the French forum: http://musescore.org/fr/node/25446

In guitar music, it is commom to merge noteheads for unisons in different voices, even in cases where there one note is dotted and the other is not, and perhaps even in cases when one note is stemless (eg, a whole note). While it seems clear this should not be the default, it should not be difficult to achieve this result. One possibility is to add an option - either a style option, staff type parameter, or note property - to control behavior in such case. But lasconic suggests a clever scheme in which MuseScore agrees to merge noteheads it otherwise would not if you simply set one explicitly to match the other, as opposed to leaving both on Auto. For instance, a dotted quarter and an eighth in unison would be merged if you explicitly set the notehead for either to "Quarter".