Make groups for selection should use marked columns as well as active region

The 'make groups for selection' command currently only uses the alignment columns intersecting the currently active region to subdivide groups. This means the only way to mark several columns for subdivision is to mark them, hide everything but those columns, then select the columns in one contiguous active region. Instead, the make groups for selection should use all marked columns in order to construct the keys used for subgrouping the alignment.