Copy or move contacts between groups by dragging

When user holds Shift on drag start, no popup menu appears and
contact is moved from source to target group.
When user holds Ctrl on drag start, no popup menu appears and
contact is copied to target group.
When user holds no modifier on drag start, a popup menu appears
when contact is dropped, allowing user to choose whether to
move or copy contact to target group.

REVIEW: 108432