Add child functions to Account in python bindings.

Add children discovery functions to Account type for the python bindings.

Author: Hendrik van Antwerpen <hendrik@van-antwerpen.net>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22806 57a11ea4-9604-0410-9ed3-97b8803252fd