Fix handling hscroll during drag-and-drop

Fix handling hscroll during drag-and-drop

* lisp/x-dnd.el (x-dnd-hscroll-flags): Delete function.
(x-dnd-get-modifiers, x-dnd-wheel-modifier-type): New functions.
(x-dnd-mwheel-scroll): Use that to determine hscroll instead.
* src/window.c (set_window_hscroll): Mark window as needing
redisplay if the hscroll really changed.