xmp: Finish two-side full-page examples

Full-page examples can now be printed starting on the correct page (odd/even) and use supplied pdf files.
We can support
- one-sided layout
- two-sided but indifferent layout (files for -odd and -even start are provided)
- two-sided explicit layout (one -odd or -even file is provided)
The last option doesn't work yet, see #23

Fix #10