Improve group membership display list

Currently, the information on how a contact arrived at a (particularly: smart) group and what can be done to it is sub-optimal. From an improvement proposed by Chris Mott in CRM-1785:

One idea for the membership list page:

• list ALL members who are selected in the query and/or have been manually added
• add two new columns to indicate why contacts are included and their status: one column would have an icon indicating if the individual matches the query results, and a second column would have a '+' or '' icon indicating if they have been manually added or removed. (there would be three possible combinations, as just '' would never occur)
• the formatting of the removed contacts rows could be changed to grey make their removed status clear