Allow "modifier none" in i3bar to disable the modifier.

This allows "modifier none" (and "modifier off") for the bar config
in order to disable the modifier key altogether. This is useful
for users who use a different approach to hiding / showing the bar,
e.g., a custom keybind that involved multiple keys or scripts.

fixes #2208

i3bar/src/xcb.c

@@ -1094,7 +1094,7 @@ void xcb_chk_cb(struct ev_loop *loop, ev_check *watcher, int revents) {
             DLOG("received an xkb event\n");
 
             xcb_xkb_state_notify_event_t *state = (xcb_xkb_state_notify_event_t *)event;
-            if (state->xkbType == XCB_XKB_STATE_NOTIFY) {
+            if (state->xkbType == XCB_XKB_STATE_NOTIFY && config.modifier != XCB_NONE) {
                 int modstate = state->mods & config.modifier;
 
 #define DLOGMOD(modmask, status)                        \