Changing of focus in the SV during pasting of sequence

Fix the following use case:

1. Open several sequences in one Sequence View.
2. Set focus to the first sequence.
3. Paste a sequence that is loaded for several seconds.
4. Set focus to the second sequence in the Sequence View.
   Expected state: the pasted sequence is appended to the first sequence in the SV.
   Bug state: the pasted sequence is appended to the sequence in focus.