Add management of multiple SeekHead Element

[gwen-lg]

Some Matroska files have a multiple SeakHead Elements, like a first one who only indicate the position of an SeekHead Element at the end of the file.

[hasenbanck]

Thank you for the PR. I think a test for this change would be helpful to understand it's implementation better.

[hasenbanck]

PR looks fine so far. Can you please add some sort of test for this?

[gwen-lg]

You think of a mkv file with multiple seekhead ?
I'm not sure if I could create a small test file with this particularity.

[hasenbanck]

I thought this change was needed to support multiple SeekHead elements. So we should be able to either create the binary stream that implements such multiple heads, or create a test file to do so. Implementing this feature without a test case is not a good idea.

[gwen-lg]

I'm able to create a file with two seekhead.
The first one (at start of the file) contain only the information (position) of the second one, who are at the end of the file.
It's not as small as the file simple.mkv, but its size seems reasonable to me (570K).

[hasenbanck]

Yes, the test file and test itself looks fine. Thanks!