Selection start/end boundaries can't be adjusted after 'Hide Insertions'

Followon from JAL-2291. Selecting a region where the first or last columns are all gaps and then applying 'Hide Insertions' will hide the columns but not adjust the extent of the selected region.

The UI doesn't allow the user to drag the boundary of the selected region if it starts or ends at a hidden column - as a result, the group becomes 'locked'.

Locked start/end positions on groups could be considered useful since it preserves the group dimensions (that's how we often explain the hidden columns and rows feature, in fact), but the lack of response from the UI makes it feel counterintuitive.