Don't rely on XdndAware on frames for dropping to work

Don't rely on XdndAware on frames for dropping to work

* src/xterm.c (x_dnd_get_target_window): New parameter
WAS_FRAME.  If toplevel is a frame, set it and clear proto and
motif.
(x_dnd_send_enter, x_dnd_send_position, x_dnd_send_leave)
(x_dnd_send_drop): Remove special-cased self dropping code.
(x_dnd_note_self_position, x_dnd_note_self_drop): New functions.

(x_dnd_begin_drag_and_drop, x_dnd_update_state):
(handle_one_xevent): Handle our own frames using those functions
instead.