player: use a separate context for timeline loader stuff

Instead of accessing MPContext in player/timeline/*, create a separate
context struct, which the timeline loaders fill out. It turns out that
there's not much in the way too big MPContext that these need to access.

One major PITA is managing (and closing) the set of open demuxers. The
problem is that we need a list of all demuxers to make sure no unneeded
streams are enabled.

This adds a callback to the demuxer_desc struct, with the intention of
leaving to to the demuxer to call the right loader, instead of
explicitly checking the demuxer type and dispatching manually in common
code. I also considered making the timeline part of the demuxer state,
but decided against: it's too much of a mess wrt. memory management and
threading, and also doesn't make it clear who owns the child demuxers.
With the struct timeline decoupled from the demuxer state, it's at least
somewhat clear that the child demuxers are independent from the "main"
demuxer.

The actual changes to player/timeline/* are separated in the following
commits, because they're quite verbose. Some artifacts will be removed
later as soon as there's only 1 timeline loading mechanism.

(cherry picked from commit a0a089f6a48c2775478b544185a0ddeabf43cce8)

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