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

include/xcb.h

@@ -165,4 +165,4 @@ void xcb_remove_property_atom(xcb_connection_t *conn, xcb_window_t window, xcb_a
  * Grab the specified buttons on a window when managing it.
  *
  */
-void xcb_grab_buttons(xcb_connection_t *conn, xcb_window_t window, uint8_t* buttons);
+void xcb_grab_buttons(xcb_connection_t *conn, xcb_window_t window, bool bind_scrollwheel);