This reverts commit e7b6d21a.

Performance are really improved, ie previewing is as fast as watching a video in VLC

Removed tiny debug

It just lacks RenderPreviewWidget to query only one frame at a time.

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