Improve drag-and-drop emulation time handling

Improve drag-and-drop emulation time handling

* src/xselect.c (x_handle_selection_request): Use
display-specific pending DND time.
(x_set_pending_dnd_time): Delete function.
* src/xterm.c (x_dnd_do_unsupported_drop, handle_one_xevent):
Set dpyinfo->pending_dnd_time instead.
* src/xterm.h (struct x_display_info): New field
`pending_dnd_time'.  Make handling pending drops
display-specific to avoid interference when there are multiple
displays.