Fixed some of Gillaume's review comments from Bug #571642

Fixed some of Gillaume's review comments from Bug #571642

The following comments should be fixed in this commit:
- tp_group_update_members: add "old" and "new" in comments so we known which
  contact we are talking about.
- Always use TpHandle instead of guint when storing handles in variables.
- contact_list_store_member_renamed_cb: would be good to share the common code
  with contact_list_store_members_changed_cb in a common function

One comment about the last one: I split out common code into two new functions:
contact_list_store_add_contact_and_connect(), and the matching
contact_list_store_remove_contact_and_disconnect().  I wondered whether the
signal connection/disconnection should just be added to the _add_contact() and
_remove_contact() functions directly, but they do seem to be used in other
places without connecting to signals, so i thought it would be safer to simply
add some wrapper functions.