Add gradual transition

[Anarky]

Hello, this PR add an optional number to parse, and use it as a number of steps to gradually change the brightness.

I just saw the issue #26; in your anim branch it seems you implement a time-based solution.
I would say it's easier to use, but they are not mutually exclusive?

[Hummer12007]

Hi, does writing to the device in a lot of steps without any delay actually make any difference in comparison to just writing it once?

[Anarky]

I just did more tests, it seems to only create a transition if ENABLE_SYSTEMD is set.
With this option, writing with 100 steps takes ~200ms.
Without it, it takes 10k steps (x100) for the same time, and I'm not sure to see a transition.

I would guess systemd apply each and every value, so sometimes repeating them creates a delay, but without it the values are batched and some are skipped?

[Hummer12007]

The reason it works with systemd is that each device write with systemd incurs the overhead of a DBus call, if I remember correctly, which is rather slow, so it introduces delay. While writes to sysfs are fast, so they happen almost immediately.

[xdbob]

Indeed, talking to logind will cause a lot of overhead compared to directly writing to the device file. IMHO only a time-base solution is viable, taking advantage of the overhead of some call is a bad idea and may break for almost any reason at any point.

[Anarky]

You're right, feel free to close this PR if you don't need it :)