gnl: Add the srcpad directly to GnlObject

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 priorities in the GnlComposition tree does not allow that;
3- Not very useful in most of the cases and it complexifies quite a lot the code
   in the composition.

Conflicts:
	configure.ac
	tests/check/Makefile.am