Allow dragging files from dired to windows on the same frame

Allow dragging files from dired to windows on the same frame

* doc/lispref/frames.texi (Drag and Drop): Document new
parameter to `x-begin-drag'.
* lisp/dired.el (dired-mouse-drag): Utilize new parameter.
* lisp/term/haiku-win.el (x-begin-drag): Add new parameter.
* src/xfns.c (Fx_begin_drag): New parameter
`allow-current-frame'.  Fix typo and update doc string.
* src/xterm.c (x_dnd_get_window_proto): Respect
`x_dnd_allow_current_frame'.
(x_dnd_begin_drag_and_drop): New parameter
`allow_current_frame'.
* src/xterm.h: Update prototypes.