In osu!stable, only the hitobjects which ended at the distance object's EndTime would be considered for further pattern generation.
Previously this generator was group _all_ objects including those that don't end at the object's EndTime, resulting in incorrect hitobject count for further pattern generation.
Also restructures with the addition of a ManiaBeatmap which holds definitions for "groups" of columns. At the moment these are empty save for a "Column" property, but can be expanded in the future, maybe.
