removed destroy handlers I was setting on items in the hash...
Mainly  because destroy  signals  are emitted  on  groups, before  the
signals on the items they contain.

Possibly more need to be removed....
revcomp...

This doesn't appear to get overwritten by other destroy handlers set on the same item.

maximise scroll region as soon as possible so other functions requiring it work (DNA)
slight order change in WindowDestroy

will now pick the most preferred font from the list (first member)

fix two problems in block positioning: must allow for no columns in a strand set, blocks within one alignment must not be bumped over.

invisible default insensitive colour!