Support exporting data from the AB into an alignment format

There is currently no way to export data from the Assembly Browser into an alignment format such as multi-FASTA with gaps. In some scenarios this may be useful. See, for example, http://ugene.net/forum/YaBB.pl?num=1541689817. However, some limitation should be added as it is not possible to add as many data in the Alignment Editor as in the Assembly Browser.