Only grab scrollwheel buttons if necessary.

With this patch, we only grab the scrollwheel buttons (4 and 5) when
managing a window if a whole window key binding exists for these buttons.

This allows both of these usecases:
  - Bindings to scrollwheel buttons using --whole-window (see #1701).
  - Scrolling in a window without focusing it if no such binding
    exists (see #2049).

Furthermore, we drop all button grabs and regrab them after a config
reload in order to reevaluate the new bindings correctly.

fixes #2049

src/manage.c

@@ -168,7 +168,7 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
     cwindow->id = window;
     cwindow->depth = get_visual_depth(attr->visual);
 
-    xcb_grab_buttons(conn, window, (uint8_t[]) {XCB_BUTTON_INDEX_ANY});
+    xcb_grab_buttons(conn, window, bindings_should_grab_scrollwheel_buttons());
 
     /* update as much information as possible so far (some replies may be NULL) */
     window_update_class(cwindow, xcb_get_property_reply(conn, class_cookie, NULL), true);