Handle scrolling during XDND drag-and-drop

Handle scrolling during XDND drag-and-drop

* lisp/x-dnd.el (x-dnd-get-object-rectangle): Handle cases where
`posn-x-y' is nil.
(x-dnd-modifier-mask, x-dnd-hscroll-flags, x-dnd-note-click):
New functions.
(x-dnd-click-count): New defvar.
(x-dnd-handle-xdnd): Handle button press events.

* src/xterm.c (x_dnd_send_position): Fix handling of mouse
rects.