Fix T90737: VSE adding nested strips could have non-unique names

Caused by {rBbbb1936411a5}.

When adding strips via the new SEQ_add_XXX_strip functions, the
`Editing->seqbasep` pointer was passed around.
Following in `seq_add_generic_update` this `seqbasep` pointer was used
to ensure a unique name.
But `seqbasep` is the pointer to the current list of seq's being edited
(**which can be limited to the ones within a meta strip**).

We need unique names across all strips though (since these are used for
RNA paths, FCurves as reported), so now use the scene's `Editing-
>seqbase` (**which is the list of the top-most sequences**) instead.

Unfortunately this might have screwed files to a borked state, not sure
if this could easily be fixed...

Maniphest Tasks: T90737

Differential Revision: https://developer.blender.org/D12256

source/blender/sequencer/intern/strip_add.c

@@ -99,7 +99,7 @@ void SEQ_add_load_data_init(SeqLoadData *load_data,
 
 static void seq_add_generic_update(Scene *scene, ListBase *seqbase, Sequence *seq)
 {
-  SEQ_sequence_base_unique_name_recursive(scene, seqbase, seq);
+  SEQ_sequence_base_unique_name_recursive(scene, &scene->ed->seqbase, seq);
   SEQ_time_update_sequence_bounds(scene, seq);
   SEQ_sort(seqbase);
   SEQ_relations_invalidate_cache_composite(scene, seq);