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First marshilling it to the main thread is dangerous as it is a blocking
operation and it should never happen there.

The asset cache is MT safe now so it is possible to load the timeline
from that thread directly

Making sure to have 1 GstDiscoverer per thread.

Use that new feature in gesdemux by loading the timeline directly from
the streaming thread. Modifying the timeline is not supported allowed
anyway.

And use it in both gesdemux and gessrc

And remove useless .h files

Fixes #64

gesdemux.c:297:3: error: value computed is not used [-Werror=unused-value]