power: Remove org.gnome.SettingsDaemon.Power D-Bus interface

It was only used by gnome-shell, which now uses UPower directly.

https://bugzilla.gnome.org/show_bug.cgi?id=709736
power: Remove use of gpm_device_to_localised_string()

It was only ever called with a UPS device.

https://bugzilla.gnome.org/show_bug.cgi?id=709736
power: Remove unused helper functions

https://bugzilla.gnome.org/show_bug.cgi?id=709736
power: Use new "warning-level" property in UPower

https://bugzilla.gnome.org/show_bug.cgi?id=709736
power: Remove unused policy GSettings

The percentage and time levels to use for low battery, critically
low battery and when to take action is now handled within UPower.

https://bugzilla.gnome.org/show_bug.cgi?id=709736
power: Use up_client_get_critical_action()

To get the configured critical action.
power: Remove impossible actions

We never do "nothing" or "suspend" when on low battery, as it
completely defeats the point. Remove the cases that handle that.
power: Don't do an action on critical battery

UPower does that for us.
power: Remove manager_critical{_ups,}_action_do_cb()

They didn't do anything different from the existing functions.
power: Naive replacement of our custom composite device

Remove the now unneeded engine_update_composite_device()
and use the display device instead.
power: Remove unused error checking
power: Use icon-names from UPower

Instead of our own.
power: Remove unused gpm_upower_get_device_icon()

We're using the icons from UPower now.
power: Simplify checking for the warning-level

Instead of capturing the old state, the old warning-level,
we now only want changes from the composite device, and
the non-battery, non-UPS devices, and just wait for
UPower to tell us that the warning level changed.
power: Simplify engine_ups_discharging()

It's only ever called for UPS devices.
power: Remove engine_just_laptop_battery()

Checking if there's only a laptop battery is as easy as
checking if there's no other devices, now that the devices_array
doesn't contain batteries or UPSes.
power: Remove "on AC" check from warning-levels

UPower already does this for us, to avoid warning levels being set
when the batteries didn't notice the AC yet.
power: Stop the alert sound before taking action

As it's UPower taking action, make sure that we stop the alert
sound before the action is taken. We might wake up to the sound
otherwise.
power: Overriding GSD_ACTION_DELAY doesn't do anything

There's no need for the test suite to override GSD_ACTION_DELAY,
action is now taken on the UPower side.
power: Set on_low_battery when the warning level changes