Make responding to selection requests work inside popups

Make responding to selection requests work inside popups

* src/xfns.c (Fx_file_dialog):
* src/xmenu.c (x_menu_wait_for_event, create_and_show_popup_menu)
(create_and_show_dialog, x_menu_show): Defer selection requests.
* src/xselect.c (x_get_foreign_selection)
(x_handle_selection_notify): Add some more info to selection
trace.

* src/xterm.c (x_defer_selection_requests): Make non-static.
(x_release_selection_requests_and_flush): New function.
(x_dnd_begin_drag_and_drop): Use DEFER_SELECTIONS instead.
(x_wait_for_cell_change): Fix initial value of rc for pushed
back events.
(handle_one_xevent): Allow GTK to respond to selections in its
windows too.

* src/xterm.h (DEFER_SELECTIONS): New slug of code.