artoolkit: patch headers to use relative includes

This enables us to move the headers into the artoolkit folder in the
devel space, which is consistent with other ROS packages and avoids
conflicts with system-wide installations of artoolkit.
artoolkit: fix install locations

move the headers to devel/include/artoolkit/AR/...
artoolkit: cleanup CMake

avoid the empty library targets which give CMake warnings, instead:
 * use the add_library(... IMPORTED GLOBAL) command, which was made
   for this situation
 * remove the additional copy targets
 * and move everything into a nice loop
ar_pose: add CMake dependencies on artoolkit_EXPORTED_TARGETS

necessary since artoolkit is built outside of CMake's control, so we
need to wait until that is finished
ar_pose: update include paths

... to use the new locations