Branching point notifies can lose their linked montage

Currently UAnimSequenceBase::OnModelModified assumes that a notify is linked to a sequence and not a montage. This causes branching point notifies to lose their linked montage when being updated and so their branching point status is also lost.

1. Make a montage from any animation and open it in the montage editor.
2. Place any branching point notify state. Note the branching-point icon on the notify state.
3. Change the starting time on the animation sequence.
4. The branching point notify is no longer marked (no icon) as such as the linked montage value has been lost