Caused by rBbbb1936411a5: VSE: Refactor VSE strip loading code.

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...