Return the correct action from the Lisp side of drag-and-drop

Return the correct action from the Lisp side of drag-and-drop

* lisp/x-dnd.el (x-dnd-handle-drag-n-drop-event): Select
`window' when handling internal DND events.
(x-dnd-handle-unsupported-drop): Return an appropriate action.
* src/keyboard.c (kbd_buffer_get_event):
* src/termhooks.h (enum event_kind): Delete
`UNSUPPORTED_DROP_EVENT'.
* src/xterm.c (x_dnd_send_unsupported_drop): Set flags instead
of recording input event.
(x_clear_dnd_monitors): Rename to `x_clear_dnd_variables'.  Also
clear unsupported drop data.
(x_dnd_begin_drag_and_drop): Run unsupported drop function
inline (and safely), and use its return value if it returned a
symbol.
(syms_of_xterm): Update doc string of
`x-dnd-unsupported-drop-function'.

* src/xterm.h: Update declarations.