Support for playlist version check

[Wkkkkk]

This first commit is a trial implementation for version checking and could be used for discussions.

[tobbee]

Let's have a discussion on where we aim.

I think I would rather like a method like "CalculateMinimalVersion()" on the MasterPlaylist and MediaPlaylist. That method could then be added to the Playlist interface.

We can then just run through the steps for minimal version checking for the two types of playlists, and every time it increases we save a reason why it increased. At the end we should have the reason for version that is returned.

In the reader, we should not have any automatic check for version, but let the user check at the end using the method if he/she finds it important.

Of course, we could do something with "strict" parsing, but that mode is not generally implemented in a good way, so I don't think it is worth the effort.

Before encoding, it may be useful to be set the right version, so that is a more relevant case, in my view. Still, I think it should be a manual test, and not automatic.

Regarding the current code in the PR, I think it is more complex than it needs to be. In my view, we just need some loops to check the conditions one by one and reporting a version and a reason.