Fix Adobe XMP imports in 'Samples'

[imagingbook]

Fixes issue #713:

• Updated Adobe XMP imports in XmpSample.java.
• Modified POM to add Samples as an additional test source directory using build-helper-maven-plugin. This causes code in Samples to be compiled during Maven's test-compile phase (as a validity check) but does not include it in the distribution.

Other suggestions:

• The current artifact version (2.19.0) is the same as the last release version - perhaps change to something like 2.20.0-SNAPSHOT?
• In the long run it may help to restructure the project to Maven's standard layout.