Prevent engine's guile dependency from leaking to modules not directly
using guile

For this I have split engine-helpers.h in two headers
- one for conversion functions to and from guile
- one with guile-independent helper functions

The appropriate headers are then included depending on which functions
are required.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22902 57a11ea4-9604-0410-9ed3-97b8803252fd