A new approach towards behavior graph export.

* [QSIM,SC] Removed all export-related code. Now in QSIM_BG and QSIM_WEB.
* [QSIM_BG,NEW] Added export_bg/3, which creates a GIF directly (not exproting an extra RDF graph). I am very happy with this approach.
* [QSIM_EXPORT,SC] Removed code for exporting behavior graph. Now in QSIM_BG and QSIM_WEB. This module is to be phased out eventually.
* [QSIM_WEB] Rewrote behavior_graph_web/2 as bg_web/3. Uses the new statement printing methid in PGC.
* [QSIM_BG,QSIM_ENGINE,FIX] Allow the use of HTML-escaped angualr brackets in GV HTML label output.