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... and render is stopped when there is nothing more to do

This way, the TrackWorkflow can return a black output when nothing has
to be rendered

However, they must NOT be simultaneous (for now !)

Removed debug that I (obviously) merged badly in a previous commit

This is breaking compilation, you can blame me, ho yeah you can !

It now starts, load, and pause all by itself, without any intervention
of the TrackWorkflow, nor anything else.
Added some debug to detect a crash, the problem is probably inside of
the metadatamanager.

The aim is to launch a clip workflow, and let it start the playback, but
pause as the first frame.

This has only be tester with a Clip end date, not with an "unset" Clip
end.
Removed debug.

There's still a size/chroma/pitch/something_else problem, but this should work
soon :)

Corrected Clip workflow, seems kinda functionnal, but will need more
testing...

VLC config files aren't loaded anymore. ("--ignore-config")

Library is now made of a GUI representation, and a frontend/backend,
that communicate with slots/signals
Also, VLCException must now be catched using the macro
CheckVlcppException, so that it prints out file and line of the error.

Please do not use 'this' pointer when it isn't required.