While more complicated, this fixes the sliders having different length
and positions.

Users can still control the sleep mode via the hidden option dpms-sleep-mode ("Standby" or "Suspend")
The default value is "Suspend" (implicitely set in xfpm-dpms.c)

The exact intent of this feature is unclear, its implementation
unfinished.

Remove all related GUI widgets from the settings dialog.

Brightness handling is usually done by the kernel, so the user should have the
privilege to decide whether power manager takes over the handling or not. It is
not good to automatically take over the handling, break it and then make the user
worry how to get everything going again.

This means we will need more options that will not fit on the display tab anymore,
so create an extra tab for brightness.

As an added benefit, this solves the layout problems with the sizing behaviour
of the sliders in the table.

...so it stops complaining.

Let xfpm control X11's screensaver extension FIXME: dbus_init in power.c resets the configured values if xfpm restarts

Can still be set through xfconf

Remove old icons which aren't used in UPower and add new icons
that are.