Allow loading of menu files outside of your XDG_CONFIG_HOME (Fix bug 5711)

When the given file name can not be found in your XDG_CONFIG_HOME, ie in
 ~/.config/openbox, then try the file name directly.

This means if you specify a menu file such as "/home/dana/helloworld.xml",
openbox will try, in order:
1) ~/.config/openbox/home/dana/helloworld.xml
2) /home/dana/helloworld.xml

And it will load the file you meant when it tries the second one.