xmp: Add two-page output (Trial)

Got stuck and therefore created this branch
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
xmp: Fix two-sided issue

Fixes #23
Now a full-page music example for which only a specific -odd or -even pdf is provided
reliably starts at the next odd/even page.
If necessary it launches a second \afterpage instance.

The problem was that I did the checks and \renewcommands within the first \afterpage block,
which obviously doesn't work.
So now I do all checks first and launch \afterpage then.
Merge branch 'twopage' into master

Realize the conditional printing of full-page examples on odd or even pages.