Move backlight handling to phosh

g-s-d 49 dropped the backlight handling so we need to do it instead

This gives us basic support bringing the slider back to live.

We still need to

• handle keybindings (see !1775 (merged))
• show osd in that case (see !1775 (merged))
• interaction with g-s-d's automatic brightness
• Drop backlight handling DBus bits that we have to make g-s-d not crash (will happen past 0.50)
• Handling of the non-logind case (see plugins/power/gsd-backlight-helper.c in g-s-d < 0.49)

@arun-mani-j could you have a look so we can get this in place for 0.50's RCs to not regress?

Closes: #1261 (closed) Signed-off-by: Guido Günther agx@sigxcpu.org