Avoided segfault when playing track two times

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

Removed debug

YOUHOU, FIRST CODE COMMIT, YAY SACRIFICE PONIES!
(Don't be afraid, I love ponies, it's a friendly sacrifice)

Added some doc

You can now play/pause the render preview

Removed useless Track.cpp/h

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

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

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 :)

It just lacks a few graphic polish (for the sliders)

This seems to breaks a few things (or at least I observed crash that didn't
occured before, but maybe that's VLC 1.1 :D

Corrected crash when launching
Added render preview widget in UI
Deleted unused VLCMediaList

There's now a central widget that dispatch preview to a ClipPreviewWidget class
if it's a clip In a close future, there will be a RenderPreviewWidget class

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