Use ARDOUR::user_config_directory in Configuration::*instant_xml

Use ARDOUR::user_config_directory in Configuration::*instant_xml

Stateful::add_instant_xml was modified so it would create the user
config directory if it doesn't exist, create_directories may throw
if it cannot be created. This isn't a change in behaviour as
ARDOUR::get_user_ardour_path also throws an exception(although I
can't see any existing code to handle it)

git-svn-id: svn://localhost/ardour2/trunk@2043 d708f5d6-7413-0410-9779-e7cbd77b26cf