Fixed references and added an example

Some functions in Xlib.d are written using "ref Display" as parameter (in Xlib.h they are C macros), this leads to some hassle because most X functions use Display*

Before:
Display *dpy = XOpenDisplay(null);
Window wnd = DefaultRootWindow(*dpy);

After (same as C):
Display *dpy = XOpenDisplay(null);
Window wnd = DefaultRootWindow(dpy);