Better implementation for SeekTable

[AaronLenoir]

The seektable uses a weird way of handling the fact that multiple placeholder seekpoints can be added. It doesn't store them at the end of the list but just keeps a counter of how many there are. It works, but it's kind of weird.

The better solution is to use a sorted list that can handle duplicate keys. If such a thing exists.

(Actually, I assume implementing a Genereic Collection that always sorts would be kind of OK)