display and handle '~' as a special gap-character for before-start or insert-after-end of sequence

Some tools display gaps in a row before the start or after the last residue in a sequence as ~ (e.g. hmmbuild -O ). Others also employ '~' to indicate that sequence was omitted (e.g. display ~ at hidden column boundaries).

Jalview currently treats '~' as a non-gap character. It should at the very least, treat ~ as a gap character, but also allow users to display or output alignments with ~ symbols displayed or omitted as desired.