Limit relationship target contact to a group of contacts in advanced search

This patch adds a select field for "Target Contact(s) in Group" to the Relationship pane. This allows the contacts on the other side of a relationship to be limited to a specified group or groups in advanced search. This allows more complex queries to be run. For example:

"Employees of organizations in New York who have given in the past month":

• Search for orgs in new york, save as a group
• search for contacts who gave in the last month, specifying "Employee of" and select the above group