This makes use of the xfconf property introduced in the previous commit.
Initially, min_level is set to -1, which causes automatic determination of
the best minimum level when the slider is created.

Setting a minimum level other than 0 might only make sense for newer
laptops, which can be heuristically determined by looking at the available
range of brightness steps. Any backlight interface providing more than 100
steps will set the minimum level the slider will support to 5, which is
barely enough light to see the screen contents. The user can change this
via xfce4-settings-editor, or its command line companion xfconf-query.

I still noticed a slow-down on my machine even with the newly introduced
update timer. Increasing it to 50ms is still enough for not causing the
updates lagging behind too much, while filtering enough events to keep
the widget response snappy even with two monitors attached.

Take the x/ythickness into account when setting the size request and
get the closest sharpest icon instead of GTK_ICON_LOOKUP_FORCE_SIZE.
Also, apply some Gtk+ style properties that the convenience functions
of the panel provide by default for panel plugin buttons.

Dragging the slider produces a lot of events, and processing these makes
the UI lag behind because of the many helper calls. Let's update the
brightness level only after a small timeout.

Thanks to Simon for pointing out that a custom menu item would
accomplish what we need and for pointing out the ubuntu app
indicator code that this patch was based on. Additional thanks
need to go to Cody Russell for creating a custom menu slider
widget I could copy.

This way it won't show as two different menu items.

This will consolidate the plugins which can then be renamed.
There's an issue where the mouse grab on the brightness slider
doesn't work, hopefully we can do that without making a custom
popup window. Scrolling on the plugin icon or over the brightness
slider does work.

When the GTK theme changes, resize the panel button and its icon.

When the user selects a device in the battery plugin, the xfpm
settings dialog is activated, the devices tab is brought to focus,
and the device is selected with the details shown.

In the battery plugin, there's a chance that we've filtered out
any devices to display and thus there's no need for the menu
separator.

This way the panel icon will increase with the LXDE icon_size
changes.

Adds support for the battery plugin to be built and used in LXDE
as the system tray icon was removed. Thanks to
https://github.com/nandhp/multiload-nandhp as it was used as a
complete example on how to easily do this. There may be a couple
things that need to be added to make this have the same
functionality as the Xfce plugin.

This makes it really obvious what the menu entry does.

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

With UPower < .99 this patch will make the battery panel icon
dynamic by displaying the ups or battery device with the highest
percentage. This patch also cleans up a couple small issues with
the last two patches (unused function and variable).

When upower reports unknown there is no useful information for the
device so present that to the user rather than a 0% battery level.

Upower 0.99 supports a composite display device that represents
the state of the computer. When that device type is around we will
use it to determine the panel icon, update the tooltip, and replace
the UP_DEVICE_KIND_LINE_POWER in the menu since it would be redundant.

The left click menu now displays the devices and their status at
the time of the menu's creation.

Rather than using a custom window migrate to custom items in a
GtkMenu. To that end, change from a GtkTreeView to GList internally.