ContextMenuEvent should be forwarded to parent
when child has Qt::NoContextMenu policy.
Here, I can't figure out what is not working,
parent never fires signal, event at viewport level.

Redirecting signal as a workaound then :/
(might be pixels offset due to missing coordinates conversion)

(cherry picked from commit 59f5a57b38e96035b8a0b918a2fc3c79b60123f6)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>