[Msa] Don't cancel rendering if user selects sequence (without changes)

Simple scenario:

1. Run the "Render overview" task.
2. While it is running, click a sequence name.
   The "Render overview" task restarted, but it shouldn't: the alignment didn't changed, just the sequence was selected.

Detailed scenario:

1. Open 10000_sequences.aln:
   1. Open UGENE.
   2. Press Ctrl+O (Cmd+O) ("Open").
      The "Select files to open" dialog appears.
   3. Choose _common_data/clustal/10000_sequences.aln.
      The AE appears.
2. The bottom line of the UGENE window displays the current task and its progress. When the opening of the alignment is complete, the "Render overview" task starts and can be seen there.
   During the "Render overview" task, select any sequence by clicking its name.
   If the "Render overview" has already completed, change the alignment, for example by removing a sequence, to run the task, and then click on any other sequence name.
   Current state: the task progress has decreased. This is because clicking the sequence name cancels one "Render overview" task and creates another "Render overview" task.
   Expected: the alignment hasn't been changed, so the task shouldn't be rerun.