This reverts commit 295c52c5.

(coming soon :p )

Added a basic media player pool

The clip QMap has been thread safed for the occasion (and to resolve a
few bugs)
Though, it seems to miss some events sometime (without crashing,
changing position is enough to restore the render preview)

Removed some debug texts

Now media isn't cloned from on vlcmedia to another, since they share
options. A new one is creater from scratch.

This also fix the crash when clicking on play without any media loaded

Added some doc

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

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.

The Main/Tracks workflow remains really basic...

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...

Removed useless stuff from VLCMedia