Gnc-Prefs: migrate int-typed preferences (and GtkCombobox widgets linked to them)

All GtkComboBoxes now use the new preferences backend
so drop the GtkComboBoxes-Gconf wiring as well.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23241 57a11ea4-9604-0410-9ed3-97b8803252fd
Gnc-Prefs: migrate file history plugin

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23242 57a11ea4-9604-0410-9ed3-97b8803252fd