Workaround for WM's not respecting (X,Y) coordinates

Hi,

Some window managers (like my own it seems: Openbox) don't take the (X,Y) coordinates into account when `xcb_create_window` is called. A workaround is to call `xcb_configure_window` after the window gets mapped.

I have only tested this in Debian/Openbox, but I can't imagine there'd be any problems in other environments.

P.S. Sorry if there's something I didn't do quite right; this is my first ever pull request!