Resave preferences dialog with a recent glade-3 version

This is in a commit of its own to separate automatic
changes from the real ones

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23244 57a11ea4-9604-0410-9ed3-97b8803252fd
Gnc-Prefs: migrate date_format preference

This includes converting the radio button group in preferences
into a combobox

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23245 57a11ea4-9604-0410-9ed3-97b8803252fd