Bug 603183 - Equity statement shows incorrect changes

Add function gnc:account-get-total-flow to report-utilities.scm

This function filters the splits from the source to the target accounts
and returns a commodity collector.
It does NOT do currency exchanges.

Author:    Carsten Rinke <carsten.rinke@gmx.de>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23156 57a11ea4-9604-0410-9ed3-97b8803252fd