In rBeaee2b411935 I removed BKE_sequencer_base_clipboard_pointers_store from
paste function, because I wanted pasted strip to always reference copied
strip, not recently pasted one. This worked well with ordinary strips, but not
if they reference ID.

Add back BKE_sequencer_base_clipboard_pointers_store call.

I thought that always referencing original strip would be more correct approach,
but it doesn't matter now. Referencing recently pasted seems to be cleanest way
to do this.