Blender Git Loki

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