Gnc-Prefs: migrate all GtkFontButton widgets (default_font property)

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

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23234 57a11ea4-9604-0410-9ed3-97b8803252fd
Gnc-Prefs: migrate GtkCheckbutton widgets (and associated preferences) - first batch

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23235 57a11ea4-9604-0410-9ed3-97b8803252fd