document multi-display support on list_screens, get_current_icc and get_current_icc_raw

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add display to print_curves for compatibility

Signed-off-by: Mattias Andrée <maandree@operamail.com>
document multi-display support in ramp getters and setters

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

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