optionally relocate sequence features after a sequence edit

If a sequence edit is made which inserts or deletes symbols from the dataset sequence, the new dataset sequence inherits the features from the original sequence without any modification, so an insert happening at position 2 effectively shifts annotation start and end positions after position 2 to the left.

Example use case is JAL-2823 - the 'Edit Sequence' dialog allows a synthetic deletion to be made, and right now, Jalview just copies the features over from the original sequence without accounting for the deleted residues. Most users would probably expect features to retain their aligned positions in the view after the deletion rather than being shifted right.

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this is related to a number of editing issues - changing start/end numbering, bulk relocation of sequence features, etc.