Improve Alignment of Annotated LilyPond N-Gram Ouptut

"Horizontal" events are now placed half-way between the "vertical" events
they connect.
Update _event_finder() in the Analyzer Module

_event_finder() now always records the offset being checked, rather
than the offset at which an event started. If somebody wishes to know the
offset at which an event started, they can use a different object-to-
string conversion lambda function.

I also updated the automated tests for this.