"Genereate distance matrix" task should be canceled on msa changing

Scenario to reproduce:

1. Open "data/samples/CLUSTALW/ty3.aln".
2. Open "Statistics" options panel tab.
3. Check the "Show distances column" option.
4. Edit the msa fast enough, e.g. insert several gaps somewhere.
   Current state: there are several "Generate distance matrix" tasks.

The second scenario:
don't edit the msa, just change the algorithm with opened distances column.

There should be only one task. Also until task has finished no values should be shown in the distances column.