redshift: Move color settings into struct for period

Move color settings (temperature, gamma and brightness) into a
struct for each period (day, night). Change the interpolation
function to interpolate all values between these structs.
redshift: Add config setting to set gamma separately for day/night

These settings are gamma-day and gamma-night. The setting gamma
will still override both the daytime and nighttime gamma setting.
redshift: Simplify set_temperature by passing color_setting_t

Changes all set_temperature function in gamma adjustment methods
to take a color_setting_t pointer with the color settings.

Colorramp functions are similarly changed to take a color settings
struct.
Merge branch 'color-settings-struct'