WINGs: fix XOpenDisplay with empty string failing for WPrefs

WPrefs.app/main.c defaults display_name to "" (empty string).
W_X11OpenDisplay passed this directly to XOpenDisplay, but
XOpenDisplay("") is not equivalent to XOpenDisplay(NULL) — the
former fails while the latter uses the DISPLAY environment variable.

Treat empty display string as NULL so XOpenDisplay falls back to
the environment variable.

Also remove debug traces from event.c and x11_backend.c.

WINGs/widgets_x11.c

@@ -249,7 +249,7 @@ WMScreen *WMCreateSimpleApplicationScreen(Display * display)
  */
 WMScreen *W_X11OpenDisplay(const char *display)
 {
-    Display *dpy = XOpenDisplay(display);
+    Display *dpy = XOpenDisplay(display && display[0] ? display : NULL);
     if (!dpy) {
         wwarning(_("WINGs: could not open display %s"), XDisplayName(display));
         return NULL;