OSX: Updated drag/drop behavior for linking an existing segment to a marker.

- Can now drag markers over segments to update them (rather than
inserting a new item). Fixed bugs in drag/drop implementation.
- Added LinkSegmentToMarker to ILoopDetailsView.
- Fixed bugs in LoopDetailsPresenter (i.e. loop is refreshed from the
database before updating its segments (was conflicting changes with
SegmentDetailsPresenter), and more.).