Improve behavior of dragging text to windows on top of frames

Improve behavior of dragging text to windows on top of frames

* doc/lispref/frames.texi (Drag and Drop): Document new meaning
of `return-frame' in `x-begin-drag'.
* lisp/mouse.el (mouse-drag-and-drop-region): Use `now' when
calling `x-begin-drag'.
* src/xfns.c (Fx_begin_drag): Update doc string.
* src/xterm.c (x_dnd_begin_drag_and_drop): Accept return_frame
as a Lisp_Object and handle Qnow correctly.
(XTmouse_position): Ignore tooltip frames when processing
`drag-source'.
(syms_of_xterm): New defsym `now'.
* src/xterm.h: Update prototypes.