Protect windows from garbage collection when a ClientMessage is pending

Protect windows from garbage collection when a ClientMessage is pending

* src/xterm.c (x_protect_window_for_callback)
(x_unprotect_window_for_callback): New functions.
(x_send_scroll_bar_event): Protect windows from garbage
collection before sending event containing pointer to window.
(handle_one_xevent): Unprotect after such a ClientMessage is
received and the window put in the keyboard buffer.
(x_term_init): Initialize protected windows list.
(x_delete_display): Free that list.
(mark_xterm): Mark the windows in that list.
* src/xterm.h (struct x_display_info): New fields for recording
a list of protected windows.