purple: workaround for buddy list menu memory leaks

purple: workaround for buddy list menu memory leaks

There is no libpurple API to release resources allocated during the
blist_node_menu() callback. pidgin-sipe allocates access levels containers
and therefore leaks memory on each invocation of the buddy context menu.

See also: <http://developer.pidgin.im/ticket/12597>

As a workaround we now keep a list of all containers allocated during the
callback. We free them when we enter the callback the next time or the account
is disconnected. So we have unused resources after the buddy menu closes but
at least we don't leak anymore...