Improve drag and drop on MS-Windows (bug#3468)

Improve drag and drop on MS-Windows (bug#3468)

Add support for 'dnd-scroll-margin' and 'dnd-indicate-insertion-point'
by calling 'dnd-handle-movement' when dragging the mouse.

* lisp/term/w32-win.el (w32-drag-n-drop): Call 'dnd-handle-movement'
when applicable.
* src/w32fns.c (w32_handle_drag_movement): New function, sends a
WM_EMACS_DRAGOVER message.
(w32_drop_target_DragEnter): Call 'w32_handle_drag_movement'.
(w32_drop_target_DragOver): Call 'w32_handle_drag_movement'.
* src/w32term.c: (w32_read_socket): Handle WM_EMACS_DRAGOVER,
sending a drag-n-drop event.
* src/w32term.h: Define new WM_EMACS_DRAGOVER message.

(cherry picked from commit 3eb2a85d10e0ad7b50e96ee4e80ba08b3a71b9ae)