gnl: Add the srcpad directly to GnlObject

Starting from now we will not claim that we support GnlObject that have
several source pads as this is 1- Not true at all; 2- The design of
priorites in the GnlComposition tree does not allow that; 3- Not very
usefull in most of the cases and it complexifies quite a lot the code
in the composition.
Some things
more violence