Fix minor bugs with XDND support

Fix minor bugs with XDND support

* lisp/mouse.el (mouse-drag-and-drop-region): Report more
selection targets for the benefit of Qt and Mozilla.
* lisp/select.el (xselect--encode-string)
(selection-converter-alist): Add new selection targets.

* src/xterm.c (x_dnd_get_window_proxy): New function.
(x_dnd_get_target_window): New argument proto_out, and return
first window with XdndAware instead of bottommost window.
(handle_one_xevent): Use new argument `proto_out'.