power: simplify brightness changed signal generation

We can use g_variant_new_parsed() for building the inner variant
and for passing the interface string too (and we save one malloc)

https://bugzilla.gnome.org/show_bug.cgi?id=729500
power: discretize the set backlight value

Most firmware backlight devices only allow a limited number of
backlight settings (around 10 or 15), but we previously sometime
failed to map the value from outside onto the discrete set of allowed
percentages.

https://bugzilla.gnome.org/show_bug.cgi?id=729500
power: signal a backlight change after initializing the rrscreen

Similar to what we do for the kbd backlight, send a fake backlight
changed event, to avoid the case when we get a dbus call from
gnome-shell before we initialized, causing the slider in the menu
to be 0 until the next change.

https://bugzilla.gnome.org/show_bug.cgi?id=729500