Move some X11 drag and drop processing to Lisp

Move some X11 drag and drop processing to Lisp

* lisp/term/x-win.el: Set unsupported drop function.
* lisp/x-dnd.el (x-dnd-handle-unsupported-drop): New function.
* src/keyboard.c (kbd_buffer_get_event): Handle
UNSUPPORTED_DROP_EVENT.
* src/termhooks.h (enum event_kind): New event
`UNSUPPORTED_DROP_EVENT'.
* src/xterm.c (x_dnd_send_unsupported_drop): Send those events
instead.
(x_dnd_do_unsupported_drop): Move actual unsupported drop
handling here.
(syms_of_xterm): New variable `x-dnd-unsupported-drop-function'.
* src/xterm.h: Update prototypes.