info: list adjustment method identifiers

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add libgamma_is_method_available

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add the names of the connector types to their doc

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m translate_from_64

Signed-off-by: Mattias Andrée <maandree@operamail.com>