pass plugin-specific OSD event handlers as arguments to createOpenSeadragonInstance

[markmatney]

It would be nice if there was a more elegant way to add extra event handlers to OSD. Currently, this is possible only by rewriting the entire method. Perhaps there should be a default list of event handlers defined as a property on the viewType (e.g. ImageVIew.osdHandlers) which could be appended to as necessary.