This commit introduces two new options:
1) Configuring the amount of brightness steps
2) Whether to distribute the steps in a linear or exponential way

Both options are presented in the settings dialog and affect the
brightness keys and the panel plugin.

This hopefully makes the icon stand out a little more.

This means we're now consistent with other panel plugins like pulseaudio or
notifyd.

Preparation for xfce4-panel 4.16

Some are related to Gtk3.22, some are simply compiler warnings.

The label will only be hidden if the display device, i.e. the one
showing up in the panel is the desktop itself.

Unfortunately UPower does not differentiate between desktop and laptop
systems so we decide that it's a desktop if it:
 * is not a UPS
 * does not have a battery
 * does not have a lid
In this case we show the "ac-adapter" icon for the systray and the
"ac-adapter-symbolic" icon for the Xfce Panel Plugin.

Before the "battery-missing" icon was shown, which is what UPower
reports for the display device.

So far the only option we have there is the formerly
hidden option show-panel-label.

DEVICE_STATE_UKNOWN means that there is no battery available.

Signed-off-by: Christian Hesse <mail@eworm.de>

The systray apparently can't use the dbus inhibt interface at all,
so use a direct call to get the list of inhibitors to display in
the menu.

Otherwise we may end up waiting for another part of xfpm that
hasn't loaded yet.

This patch now displays the applications that are inhibiting xfpm
from doing power management in the plugin/systray menu.

Found a couple spots where we inadvertently leak memory.

Signed-off-by: Eric Koegel <eric.koegel@gmail.com>

This brings back the about dialog for the panel plugin and updates
it for the newer stuff required by GTK3.

The property /xfce4-power-manager/show-panel-label has to be added to the channel
xfce4-power-manager as INT. Then you can set the value from 0 to 3, where
0: hides the label
1: shows the percentage
2: shows the remaining time and
3: shows time and percentage.

The format is not configurable for now and the label is hidden when UPower reports
the status of the display device as "fully charged".