New port: accessibility/gammy as alternative to accessibility/redshift

Gammy is a GUI tool for adjusting pixel brightness and temperature automatically
or manually.

It can dim the screen if its content is too bright, or brighten it otherwise.
This can help your eyes adjust when switching between dark and light windows,
especially at night or in suboptimal lighting conditions.

WWW: https://getgammy.com/

Submitted by: Mina Galić
Differential Revision: https://reviews.freebsd.org/D27462