Bug 710979 - Crash in gnc_plugin_page_invoice_summarybar_position_changed

Signal handlers didn't get removed properly from gsettings keys
when an invoice page was closed, leading to a crash if the
signal handler is later activated again.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23356 57a11ea4-9604-0410-9ed3-97b8803252fd