Allow dragging and dropping multiple actions

Allow dragging and dropping multiple actions

* doc/lispref/frames.texi (Drag and Drop): Document new meaning
of `action'.
* lisp/term/haiku-win.el (x-begin-drag): Correct for new meaning
of `action'.
* src/xfns.c (Fx_begin_drag): Handle new alist meaning of
`action'.
* src/xterm.c (x_dnd_begin_drag_and_drop): New parameters
`ask_action_list', `ask_action_names' and `n_ask_actions'.
* src/xterm.h: Update prototypes.