drop down menu in new feature dialog to select from existing feature types

The new feature dialog in Jalview currently only remembers the last feature group and type created on the alignment. This means that when the user is annotating sequences with a range of different types, they have to enter their desired feature type each time they annotate with a different type to the last, which is painful when interactively annotating sequences.

It would help if the new/amend feature dialog provided a drop down list of alternatives based on those available on the alignment.

Suggested by Nicole Norris: http://www.jalview.org/pipermail/jalview-discuss/2012-October/000876.html