Add "Select alignment region" button and menu item

1. Add "Select alignment region" item to the Alignment Editor context menu for sequences area.
2. Add "Select alignment region" item to the"Actions" main menu.
3. Add "Select alignment region" button to the toolbar (like "Select sequence").

The opened dialog should have coordinates and the list of sequences as in "Extract Selected as MSA" dialog, but it shouldn't be possible to close the dialog if the sequences are not in consecutive order. Disable the "Select" button and show the following message in this case:

Warning: only consecutive sequences can be selected!