(foreach_window): Instead of a fake variable argument

(foreach_window): Instead of a fake variable argument
list, take one USER_DATA argument.
(foreach_window_1): Likewise, and call callback functions with two
args, the window and USER_DATA.
(struct check_window_data): New struct.
(check_window_containing): Use it.
(window_from_coordinates): Set up a struct check_window_data for
foreach_window.
(add_window_to_list, freeze_window_start): Change parameters
according to new calling convention.