nle: Seek the whole stack on initialization

Instead of seeking each nleobject separately to setup new stack, wait for the whole stack to preroll and then seek that newly setup stack, leading to the same code path and seek 'tweaking' as when processing a seek on the composition (without stack changes).

This is mandatory to properly handle filter that tweak segments to handle time remapping for example.

cc @meh

Merge with gst-devtools!158 (merged) and gst-integration-testsuites!54 (merged)