nlecomposition: Start task and initialize the stack after chaining up to parent's...

nlecomposition: Start task and initialize the stack after chaining up to parent's change state function

Otherwise we could set the state of the children to PAUSED already (i.e.
start dataflow) from the composition's task, while the composition
itself is currently chaining up to the parent class' change state
function and did not activate the pads yet. This causes buffers and
events to be discarded, and everything to stop with a not-negotiated
error.

https://bugzilla.gnome.org/show_bug.cgi?id=774480