Power plugin: add backlight troubleshooting settings

Adds the "backlight-helper-preference-order" and
"backlight-helper-force" dconf
settings to the CSD power plugin.

backlight-helper-force causes CSD to always use the csd-backlight-helper
plugin. backlight-helper-preference-order determines the list and order
of backlight interfaces passed to csd-backlight-helper.

Together these two settings allow a user to bypass a buggy xbacklight
implementation that will prevent backlight control working on a laptop,
when the /sys interface will frequently work fine.

This allows users to resolve persistent issues on many laptops.