Bug 729157 Bill Term discount days are allowed to be more than due days

Fixed function verify_term_ok to check that discount days are no more
(or later) than due days

	modified:   src/business/business-gnome/dialog-billterms.c
Avoid defining the same translatable string twice
Bug 692249 - Add Help button in Custom Reports dialog box

	modified:   src/gnome-utils/gnc-ui.h
	modified:   src/report/report-gnome/dialog-custom-report.c
	modified:   src/report/report-gnome/dialog-custom-report.glade
Fix regression in invoice pre-payments

It was no longer possible to enter pre-payments as of 2.6.2