[Trunk] Allow chordname entry on beats

MuseScore 1.1 introduces chordnames entry on beats. Harmony elements are out of segments and have a given tick. The goal is to be able to enter quickly this kind of measure :

Having a tick for a chordname lets MuseScore exports harmonies nicely in MusicXML.

Currently in the trunk, chordnames are, like other elements, attached to segments and so can't have their own tick. The result of opening the measure below in MuseScore trunk.