Also rearrange the items to better reflect the layout in settings dialog.
It's easier to find one's way around that way.

If it's used by all components, that's the place it should be.

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.

Some laptops can turn off the backlight completely by setting the brightness
to 0. Usually, this is no problem, but in case the brightness keys do not
work for whatever reason, then the user can own himself and be left in the
dark, with no easy way to regain control of the backlight.

So as a first step, let's introduce a new xfconf property to track the
minimum value the plugin slider can be set to.

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.

Max minutes was limited to 50.

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Transifex (https://www.transifex.com/projects/p/xfce/).

Most parts in the power manager are named in different ways. Let's try
to find a more uniform naming scheme for consistency reasons. This commit
renames the UI widgets. It is only an internal change and does not
effect anything visible to the user.

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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Transifex (https://www.transifex.com/projects/p/xfce/).

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.

If this branch gets merged in the brightness plugin will no longer
be required so let's drop it now. The functionality is present in
the battery plugin.

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