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".

In deskbar mode, the button takes up too much vertical space and is not
a square. Setting an extra resize request for the button fixes it.

This reverts 0dd1196b
and adds an additional check for UPower returning "" for the icon
name.

This fixes the panel icon on a desktop system with no battery
devices.

Regression of commit f6ceafb1

and always make sure to have an icon name,
even if UPower decides not to return one

macro "g_return_if_fail" passed 2 arguments, but takes just 1.
Patch provided by Poma.

I assume this is for devices UPower doesn't know anything about?

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.