Work on DCG-based printing.

* [QSIM_BG,EXT] Adapted to DCGs, based on the DCG-improvements in the PGC and in QSIM_EXPORT.
* [QSIM_ENGINE,SC] Simplified the debug messages by using the new QSIM_EXPORT predicates.
* [QSIM_EXPORT,NEW] Complete rewrite to DCG-based printing. Also more modular/API.
Finalized the improvements to printing.

* [QSIM_BG,SC] Use of OPTION_EXT simplifies updating of indentation option values.
* [QSIM_BG,FIX] Too many newlines.
* [QSIM_ENGINE,SC] Simplified the debug messages even further by using the new dcg_debug/2 from module DCG_GENERIC.
* [QSIM_EXPORT,SC] Use of OPTION_EXT simplifies updating of indentation option values.