Per the 4.14 roadmap, Upower 0.99 is the new minimum so drop support
for older versions.

Drop old glib and xfce4ui version checks that aren't required
anymore.

It won't support GTK3 in the LXPanel. It's suggested to either
keep using the 1.4 branch or to use the systray icon once again.

After a discussion with Mattias Clasen and exploring alternative
options, we decided to switch back to GtkImageMenuItem as it works
best for what we want here.
In order for people to be able to compile without warnings, the
relevant parts have been wrapped in G_GNUC_BEGIN_IGNORE_DEPRECATIONS.

This also means the power-manager's panel-plugin is now compatible with
most modern icon-themes.

Also no more deprecated warnings.

This starts the port to GTK3. The deprecated warnings for GTK
dropping the system tray are ignored, we can deal with that in
GTK4.

This patch uses the PowerManagerButton to implement the tray icon.
It doesn't do the show when (dis)charging stuff since it will
display different battery types now, however that can be added
back if desired.

When running some versions of UPower, the code path had a minor
logic error causing it to never return the "display device". This
patch fixes that check.

Only build in the new xfce_dialog_show_help_with_version function
if the system has libxfce4ui 4.11.1, otherwise use the old help
function in order to still support systems with Xfce 4.10.

The LXPanel plugin API was completely changed in version 0.7.0
without any module versioning or backwards compatibility. This
patch make the plugin work with the new version while retaining
the ability to still build for the old version (based on checking
the package version at compile time).

There are a couple conditions where the plguin would crash or
emit runtime warnings due a device being connected or
disconnected. Those should be fixed now.

xfpm now points to the versioned docs hosted at docs.xfce.org.
This version string will always point to the major.minor of xfpm
so it will auto-increment in future relases.

When the xfconf brightness-slider-min-level is set, the plugin
will now dynamically update the range slider (if shown) for the
new minimum as well as stopping the mouse wheel from scrolling
below that level.

Since the brightness plugin merged with the battery plugin,
rename it to Power Manager Plugin. Also since we support
LXDE we'll stop calling it the Xfce Power Manager Plugin as
a LXDE plugin.

Draw the progressbar indicating the load-status of the devices
with cairo, eradicating the need for an icon per load-status.
TODO: Add the same drawing routine for the devices tab in the
settings manager.

This makes it consistent with the device icons in the same menu.

Patch provided by Simon P.

Clicking on, or dragging, the slider of the display brightness
slider of the plugin menu causes the plugin to crash. This
behaviour has been present since this feature was introduced.

I've tracked down the cause to the way the "value-changed"
signal on the scale_menu_item object is handled. The signal
is defined with a gdouble data item for the range value (but
isn't used anywhere?) while the callback function
range_value_changed_cb isn't expecting this data.

By changing to g_signal_connect_swapped for this signal in
function battery_button_show_menu & swapping the parameters
in the callback function, the range slider is now working for
me.

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.