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/config.c

@@ -229,6 +229,7 @@ void load_configuration(xcb_connection_t *conn, const char *override_configpath,
     if (reload) {
         translate_keysyms();
         grab_all_keys(conn);
+        regrab_all_buttons(conn);
     }
 
     if (config.font.type == FONT_TYPE_NONE) {