engine - move guile code to bindings/guile

This commit tries to do the minimum necessary to move the guile bits from engine
to bindings/guile. As engine is a very central piece in the software, this unfortunately
still touches many other source files:

- A few helper objects have been squashed together:
  * engine-helpers-guile.[ch] (of which the c part is extracted from engine-helpers.c)
  * gncBusGuile.[ch]
  * gnc-hooks-scm.[ch]
- The initialization function of gncmod-engine no longer initializes the scm bits.
  Any scm code that wants to interact with the engine code now has to load
  the (gnucash engine) scm module, or sometimes (gnucash business-core).
  The bulk of changes in this commit actually is updating all the scm consumers to do so.
- scm-scm target has been removed. Instead (gnucash utilities) is part
  of scm-engine. A few dependency graphs have been updated for this.

More refinements will be in followup commits.

libgnucash/tax/de_DE/tax.scm

@@ -19,13 +19,11 @@
 
 (define-module (gnucash locale de_DE tax))
 
-(use-modules (gnucash gnc-module))
 (eval-when
       (compile load eval expand)
-      (load-extension "libgncmod-engine" "scm_init_sw_engine_module")
       (load-extension "libgncmod-app-utils" "scm_init_sw_app_utils_module"))
+(use-modules (gnucash engine))
 (use-modules (sw_app_utils))
-(use-modules (sw_engine))
 (use-modules (gnucash app-utils))
 
 (export gnc:txf-get-payer-name-source)

libgnucash/tax/us/txf.scm

@@ -40,6 +40,7 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 
+(use-modules (gnucash engine))
 (use-modules (gnucash app-utils))
 (use-modules (srfi srfi-2))