set Gtk::RadioAction state on instant.xml load.

This should really be done in ArdourDropdown somehow.

Fixes issues introduced at b9708d5: If after session-load some
radio-item other than the first one is set active, it is
impossible to switch to the first.
e.g. set "Snap to Grid", Save Session, Load Session, trying to
switch to "Snap to None" fails until another mode is selected first.