WIP: Make brightness adjustment more nuanced

This is a WIP/RFC

This is WIP, please excuse the blunt g_print()s and naive code

gsd-power controls how the desktop changes brightness on key events, or when adjusting gnome-shell or gnome-control-center brightness sliders.

This API takes a value from 0-100, representing a 1:1 relation to the backlight's brightness level. This range is divided by default into 20 steps, this means that you have 20 equally spaced brightness settings to choose from.

However, the above presents a usability problem when trying to adjust brightness on the lowest levels, where you probably want a more nuanced and finer control.

For example, on my MacBookPro11,1 (2013, intel_backlight), gsd-power folows this progression:

 * value: 11 - 82 - 152 - ... - 1328 - 1388
 *  step: 01 - 02 -  03 - ... -   20 -   21

On the other hand, on its native OSX, the following progression is followed:

 * value: 22 - 37 - 56 - ... - 1140 - 1385
 *  step: 01 - 02 - 03 - ... -   15 -   16

This approach is much more usable in that it allows nuance in the lowest settings, where it is much more visible to people with sensitive eyes, or in low light situations like a dark room.

This commit introduces an ease-in curve that takes 0-100 values from the API/UI and translates them into ease'd-in values and sets those transparently to anyone using gsd-power API (shell/settings/dbus).

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Curve comparison

Note that the actual implementation follows gsd's current number of steps, and not only 16 like in the comparison

Full range of values/steps

Step	Linux	0-100		Mac	0-100		WIP
1	11	1		22	2		18
2	82	6		37	3		40
3	152	11		56	4		66
4	218	16		80	6		96
5	288	21		110	8		131
6	359	26		147	11		171
7	430	31		192	14		219
8	495	36		248	18		274
9	566	41		315	23		339
10	637	46		397	29		415
11	708	51		495	36		506
12	773	56		614	44		616
13	844	61		757	55		752
14	914	66		931	67		926
15	985	71		1140	82		1153
16	1051	76		1385	100		1385
17	1121	81
18	1192	86
19	1263	91
20	1328	96
21	1388	101