Smooth arrows navigation in MA views

1. Open Sanger alignment
2. Select a character
3. Push down-arrow button a few time until the selection goes beyond the visible range
   Current state: the row of the next selected character becomes the first visible
   Expected state: it becomes the last visible - the scroll bar moves just to visualize the next sequence

The scenario is easily noticeable in Sanger view, but is present in MSA Editor as well: just resize the sequence area the way that the last visible sequence is fully visible(no partially displayed sequence at the end).