dconf override options for backlight interface selection

This patch adds support for user dconf settings controlling how CSD chooses a backlight control interface. The usage allows forcing the use of csd-backlight-helper via "force-backlight-helper" to override the default use of the xbacklight interface, which buggily chooses the wrong control interface on many laptops (and is not easy to configure X to fix).

By adjusting the "backlight-preference-order", users can force csd-backlight-helper to choose a particular interface type, and thus get reliably functional backlight control when X11 refuses to work correctly.

I can confirm this patch solves the problem on my Dell L501x, and should work for any laptop where a working interface is found under /sys/class/backlight.

Closes #28, #17